Laboratory Manual
Laser and Optics Section
Project QCC TechASCEND
This material is based upon work supported by the National Science Foundation
under Grant No. 0206101
Any opinions, findings, and conclusions or recommendations expressed in this
material are those of the author(s) and do not necessarily reflect the views of the
National Science Foundation.
Page 1 of 77
Table of Contents
Topic
Introduction
Page
2
#1 Alignment Skills
3
#2 Refraction and Total Internal Reflection
10
#3 Lenses, Image Formation, and Telescopes
18
#4 Reflection of Polarized Light
25
#5 Spectroscopy
35
#5B Monochromaters
47
#6 Analog Oscilloscopes
54
#7 Prisms
56
#8 Beam Expanders and Spatial Filters
60
#9 Holography
66
#10 High­power Laser Demonstration
74
References
76
Budget
77
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Introduction
This lab manual is designed for a program which originated at Queensborough
Community College to introduce you to careers in technology in the areas of lasers and
fiber optics. You will be working with state­of­the­art equipment that is both costly and
fragile. Please be careful when operating any of the equipment set up for your
experiments because if you are not careful you could be injured and the equipment
could become damaged. The following guidelines will help you to use the equipment
properly and safely (See reference #5):
*When you enter the room and find the equipment set up on the lab bench, DO NOT
TOUCH anything until the instructor has explained the experiment and the safe use of
the equipment;
*If you want to try out something with the equipment, please ask your instructor for
advice.
*Do not force anything to move beyond its normal operating range. In other words, if a
knob doesn’t seem to want to turn anymore in one particular direction, do not try to
force it;
* You will receive and sign a separate sheet dealing with laser safety, but it is so
important that it will be repeated here: NEVER AIM A LASER AT ANOTHER PERSON
Please be ready to begin as soon as the session begins. If at all possible,
everyone should arrive at the lab promptly. Otherwise, you will miss the explanation
of the lab exercise for that week.
Finally, please try to attend each session.
We are here to help, so please, ask questions and enjoy the project!
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Information about careers in technology can be found at:
http://www.qcc.cuny.edu/physics/lfot/LFOT_FAQs.asp
Laboratory #1: Alignment Skills Exercise
What’s Cool About Alignment Skills?
It may not sound like much, but being able to align a laser beam is something
that most people can’t even imagine doing. Surveyors do it to measure distances much
more accurately than they could with the old equipment. Anyone working in a laser
company, building, repairing, testing, and installing laser systems must have good
alignment skills. Alignment skills even come into play if you work for a gun company
that uses laser – guided scopes! The military uses lasers to accurately aim at their
targets, so their equipment must be built and maintained by someone who knows how
to align a laser beam. Being able to align a low­power laser can also be used to get a
high­power laser up and running. Basically, almost any job in the laser field requires
good alignment skills. This lab will show you how to get started.
Background
Laser beam alignment, and alignment of optical components in general, is a very
important skill for anyone wishing to work with optical systems. “Alignment” means
getting the laser beam to follow the path that you need it to follow in order for a device
or experiment to work properly. For example, when a laser is being built, usually
another laser is used to align the new laser in order to get it running for the first time.
Alignment is also important for people using surveying equipment which can contain
lasers; in order to make a measurement, the laser must be aimed at the correct target.
Finally, if you are setting up an optical experiment involving many mirrors, beam
splitters, lenses, and other components, they all must be aligned so that the laser used in
the experiment will be properly directed through the various mirrors and lenses.
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Optical alignment requires skill, but also patience and steady hands. It requires
the use of a specific method, not just random adjustments in the hope of stumbling onto
the right position. It also requires very small corrections when the system gets close to
final alignment, so it is important not to get impatient and tweak things too much when
you start getting close! If you follow the method described in this write­up, you will
begin to develop good alignment skills. It, of course, takes a lot of practice to get
proficient at it, but every time you do it, it gets a little easier than it was the previous
time. If you have trouble, don’t give up! Take a break, then ask for help from the
instructor.
Procedure
In this exercise you will learn to use mirrors to direct a laser beam along a
specific axis and through two pinhole apertures. This is very much like the process
used to align a new laser that is being built. This experiment is divided into three parts.
In part one, you will align the laser beam to pass through two pinholes set up in a Z­
pattern. In part two, you will adjust the path of the laser beam to not only pass through
the pinholes, but to follow a straight­line Z­pattern mapped out by masking tape
between the laser, mirror, and pinholes. In part three, an extra pinhole and mirror will
be added and you will align the laser beam to pass through all three pinholes.
The experimental setup is shown below in Figure 1:
mirror 1
HeNe Laser
pinhole 1
mirror 2
pinhole 2
white card
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Figure 1 – Experimental Layout for Part One
Part One
As shown in Figure 1, you will need to manipulate two mirrors in order to make
the laser beam pass through both pinhole apertures. This is because you need to control
both the position and the angle of the laser beam. Do not make any adjustments on the
laser itself; this was set by the teacher and should be left alone. Also, do not move the
pinholes.
First, you will rotate the first mirror holder gently clockwise and counter­
clockwise to make the laser beam pass through the first pinhole and strike the second
mirror. You may also slightly tilt the first mirror within the holder to make the laser
beam go up and down a little bit, but you do not have a lot of play in this direction. If
the laser beam is set at the correct height for the pinholes, you should not need to tilt the
mirror too much. By looking at Figure 1, you can see that the horizontal adjustment of
mirror 1 will control the position of the laser beam as it strikes the first pinhole, and the
horizontal adjustment of mirror 2 will control the position of the laser beam as it strikes
the second pinhole. You must alternate between adjusting the position of both mirrors
until the beam is lined up with the both pinholes; the vertical height will only be
slightly adjusted by slightly tilting the mirrors in their holders, but again, they cannot
and should not need to be tilted too much if the laser height is set correctly. Alternate
back and forth between rotating the mirrors clockwise and counter­clockwise to get the
laser beam through both pinholes. You may also have to slide the mirrors left and right
a little to keep the laser beam on them. If you have any question as to the vertical
alignment of the laser with the pinholes, ask your instructor. Do not attempt to move
the laser yourself. Hints about alignment will be given in class and are shown below,
although you only really have to worry about it in one direction: horizontal.
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As an example, let's say that your laser beam is misaligned horizontally as follows, and,
let’s insert a second pinhole to demonstrate the alignment procedure:
mirror 2
The beam is passing through the first pinhole, but is not passing through the second.
The way to fix this alignment problem is to first raise the position of the laser beam by
rotating mirror 1 counter­clockwise and then tilt the laser beam downward by rotating
mirror 2 clockwise to obtain a level beam which should be horizontally lined up with
the bottom pinhole:
Raise:
Tilt:
**Note that the first step of raising the position of the laser beam appears to make the
alignment problem worse, but it is the only way to fix the alignment problem.** Keep
this example in mind as you work with your mirrors ­ when something is misaligned,
you must adjust both position and tilt. With your equipment, you can do both by
rotating the mirror mount clockwise or counter­clockwise, but you may occasionally
have to also slide the mirror left or right to keep the laser beam on the next mirror.
Remember to always be aware of where laser beams go when you change their
path and do not ever look straight into the path of a laser beam, even a weak one. Make
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sure your beam is not shooting across the room or onto anyone else’s table. If it is, use a
book or anything handy and safe to block the stray beam, which is not part of your
experiment, form going off of your table or assigned work area. Finally, the slight tilt in
the vertical direction is not what is meant by tilt here; this is for the horizontal
alignment only; as said before, if the set­up was initially correct, there should be no
need for massive vertical alignment.
Part Two
Once the horizontal alignment is completed, you must refine it by making sure
the laser beam follows a taped pathway that your instructor will have on the table
before the lab begins. The taped pathway will trace out a Z­pattern from the laser to the
white card at the end of the set­up. Not only must your laser beam pass through the
two slits, but it must also always be directly above the masking tape on the table. This
requires you to refine your adjustments a little, gives you some more practice, and
makes sure everyone in the lab group gets a turn. Laser beam alignment is not difficult,
but it is tedious and it gets MUCH easier if you practice it a lot. In the beginning, it will
seem to take a long time, but if you put in the effort, soon, you will be able to align a set­
up like this in minutes. Practice is the key, so if you get frustrated, remember, everyone
does at first, and you will get better if you do not give up. Take a break, let someone
else have a turn, then try again, but before this lab is over, make sure every group
member is capable of getting that laser beam through both pinholes. You will need
these alignment skills for future labs, and especially if you would like to become an
optical lab technician!
Part Three
If you have successfully completed parts one and two of the experiment, try this:
add a third mirror in place of the white card, and a third pinhole after that mirror. Place
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the white card at the end. Now, repeat the above procedures to make the laser beam go
through all three pinholes and strike the white card at the end. Make sure everyone in
the group can do this, and before you begin, “mess up” the alignment so you must start
from scratch with the first two pinholes. Again, practice, practice, practice!
Equipment List
From the Daigger Sci­Ed Warehouse Catalog, 2005
www.daigger.com, 1­800­621­7193
Catalog Location
Number
Name
Price Quantity
p. 75
DH3529A
HeNe Laser, 0.5 mW
$410.00
(1)
p.75
DH3361A
Light Box & Optics Set
$85.09
(2)
p. 76
DH3509
Economy Optics Kit
$24.00
(4)
From the Edmund Optics America Optics & Optical Instruments Catalog, 2005
www.edmundoptics.com
Catalog Location
Number
Name
Price Quantity
p. 144
G56­879
4”x4” pos. lift
$146.00
(1)
This equipment list provides the optics holders (Economy Optics Kit), round mirrors,
laser, white card with holder, slits (Light Box & Optics Set ­ slits can be taped to form
pin­hole apertures and held with holders from Economy Optics Kit). This will allow the
students to position the mirrors, in the horizontal plane only, to see how difficult it is to
get the laser to make the z­path through the slits, which are mounted on the same type
of mount as the mirrors and are therefore at the same height. This experiment can be
done for $750.18, but many of the parts are needed for other experiments, and the bulk
of the cost is the laser, which is needed in almost every other experiment, so the price
should not be fully considered until the total equipment list is finished.
Page 9 of 77
Page 10 of 77
Laboratory #2: Refraction and Total Internal Reflection
What’s Cool About Refraction and Total Internal Reflection?
Any time a beam of light passes from one material to another, refraction occurs.
Refraction is the basis for how lenses direct light, so you must understand refraction in
order to understand how lenses work. Most optical systems that use laser beams use
lenses, and in order to figure out what the lens will do to the laser beam, you need to
know about refraction. Refraction also happens in our atmosphere, so military planes
and spy satellites using detectors and guidance systems must take refraction into
account. Total internal reflection (TIR) is the reason laser light is able to travel through
optical fiber; if you want to be a fiber technician, understanding TIR is the first step. It is
a way to make a “mirror” of sorts out of almost any material, depending on the angle of
the light and the material the light is coming from. It’s not hard to understand, but
understanding it can lead you towards a career in fiber optics. TIR can also be used to
re­direct laser beams without using a regular mirror. You will do this in this lab with a
regular piece of plastic.
Refraction ­ Background
In this exercise you will learn how light bends when it goes from air into another
material, like glass or plastic. This “bending” is called refraction – it’s the reason why, if
you are standing in a pool of water and you reach for something at the bottom, like a
coin, the coin isn’t sitting where you think it ; the light bends, but your brain assumes
the light is traveling in a straight line (see Figure 1). Refraction is important because
without it, optical fibers would not work. What you will learn in this experiment
relates directly to how optical fibers are designed to transmit information.
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your view of
the coin
air
water
coin is here
you think the
coin is here
Figure 1 – Illustration of how light bends when it crosses the boundary between two
materials
Materials have what is called an index of refraction which tells us how much,
and in what direction, the light will bend when it enters that material. The index of
refraction in a perfect vacuum (totally empty space) is 1.000. In air it is close to, but not
exactly, equal to one – it is 1.0003, but for practical purposes we can use 1.0 for the index
of refraction of air. In glass, the index of refraction is usually around 1.5, and in water,
it is 1.33. Notice how the index of refraction for various materials is greater than 1. This
is because the index of refraction is defined as the ratio of the speed of light in vacuum
to the speed of light in the material. Light travels faster in vacuum than in any other
material, so that means the index of refraction must always be greater than 1 for all real
materials.
If light hits a boundary between glass and air, as shown in Figure 2, the light will
bend as it crosses the boundary – it will change directions, just like it did in the
swimming pool example. The direction of the light is described by the angle at which it
strikes the surface, measured with respect to the surface normal ( a line perpendicular
to the surface), as shown in Figure 2.
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surface normal
q1
n1
air
n2
glass
q2
your eye thinks the
light goes here
light goes
here
Figure 2­ Light coming from air and entering a piece of glass gets bent at the surface.
The real path of the light in the glass is shown by the solid line, while the path your eye
thinks the light takes is shown by the diagonal dotted line.
There is a simple way to predict how much the light will bend when it crosses
the boundary between two materials. If the light is incident from a material with index
of refraction n1 at an angle of q1 and, after it crosses the boundary into a material with
index n2 it is at an angle q2, the equation relating all these quantities is called Snell’s
Law:
n1sinq1 = n2sinq2
In this exercise you will figure out the index of refraction of a piece of glass by tracing a
light ray through it and measuring the angles q1 and q2.
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Part I – Refraction ­ Procedure
In this part of the lab, you will draw a diagram like that of Figure 2 using a piece of
lucite (we’ll call it “glass’ to make it simpler) and a laser beam. Since it is not possible to
“trace” the path of the laser beam with your pencil, you will first mark the path with
pins, then remove the pins and draw straight lines through the pin holes in the paper.
Follow these step­by­step instructions:
1. Place the glass plate on a sheet of paper over the cork board. Trace the outline of
the glass on the paper.
2. Turn on the laser and aim it at the glass so that it is passing through the front and
back surface of the glass at some angle other than straight­on.
3. Mark the beam path on the paper by inserting pins along the beam path on both
sides of the glass. Two pins on each side will be enough for you to mark the path
of the laser beam.
4. Remove the pins and the glass block and draw straight lines through the pin
holes up to, but not through, the glass block outline.
5.
Now draw the laser beam path through the glass by connecting the entry and
exit points (connect the two lines you drew in step 4).
6. Draw a line perpendicular to the glass surface at the point where the beam
entered the glass and again where it exited the glass. Your paper should now
look something like this:
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q1
outline of
glass
block
lines
drawn
through
pin holes
q2
Measure the angles q1 and q2 and, using n1 = 1.0, use Snell’s Law to calculate the index
of refraction of the glass.
Total Internal Reflection ­ Background
As you can see from Part I, when the light entered the glass, its angle with
respect to the surface normal decreased; in other words, q1 > q2. As you can see from
your drawing, when the light exited the glass on the other side, its angle with respect to
the normal increased compared to when it was inside the glass: q1 < q2. This makes
sense because when the light comes back out of the glass, it should be at the same angle
it was at when it entered (q1), so it must bend away from the normal when it exits the
glass because it bent towards the normal when it entered the glass.
The result of all of this is that when light goes from a high­index material to a
low­index material, its angle will always increase as it makes the transition. So, when
going from glass to air, if we keep increasing the angle at which the light hits the
boundary, the exit angle will have to keep increasing. This can’t go on forever; we will
eventually reach the point where the exit angle will be 90 degrees – the beam will be
parallel to the glass surface. If we increase the angle farther, the light will not be able to
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exit the glass – it will be reflected back into the glass as if the glass­air boundary were
like a mirror. This is called total internal reflection (TIR) and it is the reason that
optical fibers are able to transmit light efficiently. The “core” of the fiber contains a
high­index material and the surrounding “cladding” is made of a low­index material –
the light therefore gets trapped in the core and totally internally reflects as it travels
down the fiber.
The condition for TIR comes from Snell’s Law…we simply plug in q2 = 90
degrees and solve for q1, which is then called the “critical angle” – it is the angle above
which TIR will occur. So,
sinqc = 1/n2
if we are going from glass into air, and n2 is the index of the glass. This means that any
light which strikes the boundary at an angle greater than qc will be reflected back into
the glass. If it strikes the boundary at an angle less than qc, it will still be able to cross
the boundary.
Part II – TIR ­ Procedure
You will use TIR to find the index of refraction of a glass semi­circle and an acrylic one,
and you will see if there is any difference in your calculations.
1. Place the semi­circle made of glass on a sheet of paper over the corkboard and trace
its outline. Remove the plastic temporarily. Mark the center of the flat side of the
semi­circle. Draw a normal (perpendicular line) through that center. Put the plastic
back on its outline.
2. Aim the laser through the semi­circle from the curved side and try to hit the center
of the flat side. You should get something which looks like this:
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refracted
ray
q
incident
ray
reflected
rays
Note that, even before you reach the critical angle, there will be a reflected ray from the
flat side and from the curved side. This is normal; light always reflects a little bit when
it hits a piece of plastic or glass. This is NOT TIR though as long as the refracted ray is
still coming out on the other side.
3. Increase the angle of incidence q by rotating the paper and the semi­circle together
until the refracted ray is parallel to the flat side of the semi­circle. Make sure the
laser is still hitting the center of the flat side as you rotate! The point at which the
refracted ray starts to disappear (when it is parallel to the flat side) is the onset of
TIR. At this point, stop rotating, mark the incident ray with pins, and remove the
semi­circle.
4. Connect the incident ray to the normal on the flat side and measure the angle that
the incident ray makes with this normal (the angle marked “q” in the drawing
above). This is the critical angle qc. Use it in the equation
sinqc = 1/n2
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to calculate the index of refraction (n2) of the glass semi­circle.
5. Repeat this procedure with the acrylic semi­circle; again, this will give everyone more
practice and will allow you to see the difference in working with different materials
which may look the same, but are not scientifically the same.
Equipment List Fro m the Daigger Sci­Ed Warehouse Catalog, 2005
www.daigger.com, 1­800­621­7193
Catalog Location
Number
Name
Price Quantity
p. 76
DH9283A
Prism & Lens Set, Glass
$61.00
(1)
p. 76
DH9283B
Prism & Lens Set, Acryl. $51.00
(1)
This set, together with the laser from lab #1 and the rectangular piece of lucite from the
Light Box & Optics Set from lab #1, contains the rectangle and semi­circle required to
perform all aspects of this experiment. It also adds a prism, so this experiment may be
expanded and the prism used to further study geometrical constructs of Snell’s law (see
lab #7). You still have to find some corkboard, paper, and pins somewhere; optics
catalogs tend not to stock such things, but you can purchase them at Staples stores or
any office supply store. This lab adds another $112 and change to the equipment total,
but also provides components needed for the “lenses” lab and the “prisms” lab.
Page 18 of 77
Laboratory #3: Lenses, Image Formation, and Telescopes
What’s Cool About Lenses, Image Formation, and Telescopes?
Lenses re­direct light and it would be very difficult to form images or focus light
at all without them. You could do it with mirrors, but lenses work in almost the same
way, so once you understand one, you understand the other. Image formation is
interesting because it relates to things like holography, cameras, detectors for military
applications, photocopiers, laser printers, and of course TV and movies! This is just a
partial list of things that use lenses and images, but any kind of movie or photo
projector can be understood once lenses and image formation is understood. Finally,
telescopes use lenses (and mirrors) to help us understand our universe. Galileo created
the first telescope using two lenses, which is something you will also do. Without lenses
and image formation, our information about our world and our universe, and our
ability to record that information would not exist, and the entertainment industry
would almost not exist either! Business would not be able to print and copy documents
with the quality we have today, and life as you know it would be very different.
Knowing about how lenses form images is a very powerful skill that can help you
become an optical technician.
Background
In this exercise you will learn how lenses are used, individually and in
combination, to form images. You will also build a simple telescope and see exactly
what it does to the image of the original object. Lenses can be either converging or
diverging, depending on what they do to light rays which hit them. If parallel rays
strike a converging lens, they get focused to a point by the lens, as shown in Figure 1. If
parallel rays strike a diverging lens, they spread out after passing through the lens, but
they look like they came from a single point in front of the lens, as shown in Figure 2.
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image formed
here
incoming
light rays
Figure 1 – Image formation by a converging lens. This image is a “real” image.
incoming
light rays
image formed here
Figure 2 – Image formation by a diverging lens. This image is a “virtual” image.
The equation which tells us where an image is formed for a given lens is: 1 1 1 = +
f o i lens
object
image
o
i
Figure 3 – Diagram illustrating the quantities given in the lens equation.
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where f is the focal length of the lens, o is the object’s distance from the lens, and i is the
image’s distance from the lens. If you place the object very, very far away from the lens,
then o will be very large and 1/o will be very small, so the image distance i will equal
the focal distance f. This is the example shown in both Figures 1 and 2; an object which
is very, very far away sends light rays to the lens which are almost parallel and can be
considered parallel.
The equation above can be used for both converging and diverging lenses. If the
lens is converging, the focal length gets plugged in as a positive number; if the lens is
diverging, the focal length gets plugged in as a negative number. Then, if the image
distance i turns out to be positive, the image is located on the opposite side of the lens
compared to the object. This is called a real image. If the image distance i turns out to be
negative, then the image is located on the same side of the lens as the object. This is
called a virtual image. The difference between a real image and a virtual image is
simple: a real image has real light rays passing through it, whereas a virtual image is an
image that appears to be at a certain point in space, but the light rays don’t actually go
to that point. Figure 1 shows an example of a real image, whereas Figure 2 shows an
example of a virtual image – the rays do not actually converge on the opposite side of
the lens, but they appear to converge on the object side of the lens.
We will use different lenses and different object distances to find the focal
lengths of some individual lenses, and then we will use some lenses in combination to
build a simple telescope. The way to deal with combinations of lenses is simply to take
them one at a time. You apply the lens equation to the object and the first lens, and you
find the image location as if the second lens weren’t there. Then, you use the image
from the first lens as the object for the second lens, and you apply the lens equation
again. This means that you must measure the distance o from the second lens to the
image from the first lens (which is now your new object). The value for i that you get
this time is the location of the final image for the two­lens combination. If you had more
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lenses in combination, you would just keep on applying the lens equation to each one in
turn. Figure 4 illustrates the two­lens example.
lens 1
object
object for
lens 2
final
image
lens 2
image from
lens 1
i
o
o
i
Figure 4 – Illustration of a two­lens combination.
A simple telescope can be made with a two­lens combination. A telescope takes parallel
input rays (because it looks at stars that are very far away) and magnifies them,
producing parallel rays at the output. Because the output rays are parallel, your eye
does not visualize the image from a telescope as being at a specific location; rather, it
looks lie the planet or star is floating “out there somewhere.” You will not see this effect
in the lab demonstration because the telescope you will build will be used to view an
object close by, but if you ever look through a telescope at a planet, you will notice that
you have no idea how far away the image of the planet is!
The way to construct a two­lens telescope is simple; remember that we said that
parallel input rays focus at the focal point of the lens? Well, if you apply that theory to
both ends of the telescope, you will see that you will get parallel rays out for parallel
rays in when the lenses are separated by the sum of their focal lengths. This is
illustrated in Figure 5.
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parallel
input rays
f1 + f2
parallel output
rays
Figure 5 – A two­lens telescope. The lenses have focal lengths f1 and f2; they are
separated by a distance f1 + f2 to produce parallel output rays when parallel input rays
come into the telescope.
Procedure
Single Lens
1. Place your light source (candle or flashlight) as far away from the lens as you
can on the optical rail. Use an f = 15 cm double­convex lens and find a clear
image of the light source; measure the distance of this image from the lens; it
should be 15 cm, the focal distance, because o is large in this case. How close
is your measured value to this theoretical value? Is the image rightside­up or
upside­down? Take notes in the spaces provided here.
2. Put the 15 cm lens on the rail and put the light source 20 cm in front of it.
Find the sharpest image and measure its distance from the lens.
3. Use the lens equation with o = 20 cm and f = 15 cm to calculate what i, the
image distance, should be. Compare this with your measured result.
4.
Repeat step 3 with the light source located 30 cm from the lens.
Page 23 of 77
Two lenses
Place the light source 20 cm in front of the 15 cm lens. Place the other 15 cm lens 30 cm
behind the first 15 cm lens. Find the sharpest image formed by the second lens and
measure its distance from the lens. Use the lens equation to determine this image
distance.
If time permits, repeat this experiment with one of the 15 cm lenses replaced by the 40
cm lens.
Simple Telescope
Use the both lenses (the 15 cm and the 40 cm) to construct a telescope using the
guidelines given in the background section. If it works, you should be able to look
through the lens back towards the light source and see an inverted image of it. Is the
image larger than the source or smaller?
Diverging Lenses
1. Use the 15 cm double­concave lens and repeat the single­lens experiment and
calculations. Did you get an image this time? If not, why not? What did the
calculations tell you? A class discussion of the differences in results between
using a double­convex lens and a double­concave lens should follow these
procedures.
Page 24 of 77
Equipment List Fro m the Daigger Sci­Ed Warehouse Catalog, 2005
www.daigger.com, 1­800­621­7193
Catalog Location
Number
Name
Price Quantity
p. 76
DH3508
Meter Stick Opt. Bench
$29.00
p. 76
DH3508A
38 mm double convex lens $5.00
(2)
p. 76
DH3508C
38 mm dbl. concave lens
(2)
$5.00
(2)
All lenses have focal length of 150 mm.
From the Edmund Optics America Optics & Optical Instruments Catalog, 2005
www.edmundoptics.com
Catalog Location
Number
Name
Price Quantity
p.26
G45­298
dbl.convex 40cm fl lens
$33.80
(1)
The additional cost is $72.80, but the 40 cm lens is also needed for experiment #6 on
beam expanders and spatial filters and the meter stick optical bench and lenses are used
in numerous other experiments.
Page 25 of 77
Laboratory #4: Reflection of Polarized Light
What’s Cool About Polarized Light?
Polarized light is all around you every day. It is used in common items
such
as sunglasses, computer screens, and wrist watches, but it also has many applications
in high­tech devices like lasers, optical radiation detectors, and telecommunications
systems. Most light that reflects off of things like car hoods, windows, and the street is
polarized, so that’s why polarized sunglasses are good. They cut down on glare.
Computer screens and wrist watches that use liquid­crystal displays (LCDs) use
polarizers so that you can read the letters and numbers – the polarizer creates the
difference between the black numbers and grey background on a digital watch, for
example (see Reference #4 for more information on LCD displays and polarization).
Lasers need polarizers so that the light emitted has only one direction of oscillation
(we’ll discuss that later), which is vital for many experiments. Much research that uses
laser light requires it to be polarized. That means, not only does the laser need a
polarizer, but the optics in the rest of the experiment might need a polarizing coating on
them in order to work right. Similarly, detectors might look for light of only one kind of
polarization, which can increase their sensitivity and effectiveness, like for military uses
and other applications. Finally, fiber­optic telecommunications systems use polarized
light to cut down on loss and other complicated effects which can degrade the signal. So
you see, understanding polarization is very important for an optical, laser, or fiber­
optics technician!
Background
The purpose of this laboratory experiment is to study the reflection
characteristics of polarized light as its angle of incidence and polarization are varied.
Whenever light is incident on an optical surface there are three things which can
happen; the light can be reflected, transmitted, or absorbed. For most optical surfaces
all three of these things happen, but usually one of them will dominate. For example,
Page 26 of 77
you know that a glass window transmits light, but actually it reflects 4% of the light
which is incident upon it. Similarly, a mirror is primarily a reflecting surface, but a
very small percentage of the light is transmitted through most mirrors. For purposes
of this lab, we will be ignoring absorption.
Polarization deals with which way the electric field part of the light is oscillating.
Light is an “electromagnetic wave” which contains both electric and magnetic fields
that oscillate (move up and down) as they travel; they look like the kind of wave you’d
see if you tied a rope to the wall and shook your hand up and down. This type of wave
is called sinusoidal and looks like the wave shown in Figure 1.
direction
of travel
direction
of
oscillation
Figure 1 – An example of a sine wave; the electric and magnetic fields which make up a
light wave oscillate like this as they travel.
The wave shown in figure 1 is called a transverse wave because the direction of
oscillation is perpendicular to the direction of travel of the wave. Now, remember that
light travels in three­dimensional space, whereas the pictures we draw here are only
two­dimensional. The actual light wave can oscillate in the direction shown in Figure 1
(up and down on the page), or it can oscillate in and out of the page, or any direction in
between. If the direction of oscillation of the wave varies randomly as the wave
travels, and it does not favor any one particular direction, then the light is unpolarized.
Page 27 of 77
A polarizer is an optical component that restricts the direction of oscillation of
the wave to a particular direction. There are many ways to do this, but in this lab, we
will only be concerned with what is called linearly polarized light, which means that it
is either vertically or horizontally polarized. If the light is vertically polarized, for
example, it can only oscillate in the vertical direction. If it is horizontally polarized, it
can only oscillate in the horizontal direction. In this lab, you will see that polarization
affects the way light reflects off of a piece of glass and we can use this property to turn
unpolarized light into polarized light. Let’s now take a look at what happens when
light reflects off of a boundary between two different materials, for example, glass and
air.
The amount of reflection and transmission which occurs at the boundary
between two materials is determined by their indices of refraction (recall from lab #2).
For normal incidence, meaning the light strikes the boundary perpendicular to it, or
“straight on,” the reflectance R (defined as the reflected beam intensity divided by the
incident beam intensity) is given by
R = {[n1 ­ n2]/[n1 + n2]}2
Where n1 and n2 represent the indices of refraction for the two materials; it does not
matter which is the "entrance" media and which is the "exit" media because, as you can
see, the equation is symmetric in n1 and n2 , meaning that if you swap them it makes no
difference. So, for the example of ordinary window glass, if we use an index of
refraction n1 = 1.5 and the index n2 = 1.0 for air, the above equation tells us that the
reflectance at the air/glass interface is 4%. This means that any time light hits regular
window glass, 4% of it is reflected back rather than being transmitted through.
Fresnel's Laws of Reflection tells us that when the angle of incidence is anything
other than normal (zero degrees) the reflectance will depend on the polarization of the
Page 28 of 77
incident beam; this means that the amount of reflected light will be different for
horizontal polarization than for vertical polarization. The way the light will behave as
the angle of incidence is changed is shown in Figures 2 and 3.
Figures 2 and 3 show
reflectance as a function of angle of incidence for both “s” and “p” polarizations (this is
the short­hand way to denote vertical and horizontal polarization directions). Figure 2
represents the situation when the light is coming from air and reflecting off of the glass
surface, whereas Figure 3 shows the case where the light starts in glass and reflects off
of the boundary with the air. Notice that Figure 3 shows qc, the critical angle (recall
from lab #2). Beyond the critical angle, the light cannot escape the glass; it experiences
total internal reflection.
Reflectance air to glass Reflectance 1.2 1 0.8 R s 0.6 0.4 R p
0.2 0 0 20 40 60 80 angle (Degrees) Figure 2 – Reflectance as a function of angle of incidence for the case of light
coming from air and reflecting off of glass.
100 Page 29 of 77 Reflectance glass to air 1.2 Reflectance 1 0.8 0.6 R s 0.4 R p
0.2 0 0 20 40 60 80 100 angle (degrees) Figure 3 – Reflectance as a function of angle of incidence for the case of light
coming from glass and reflecting off of the boundary with air.
Note that in both figures 2 and 3 for p­polarization there is an angle for which
the reflectance is equal to zero. This angle is called Brewster's angle and it is defined
by the equation
tan qB = n2/n1.
Recall from lab #2 the formula for the critical angle:
sin qC = n2/n1.
Page 30 of 77
We will not be studying total internal reflection (TIR) during this laboratory exercise,
but you should recognize that TIR is the reason why the curve in Figure 3 does not
extend out to 90 degrees like the one in Figure 2.
So, we see that there is an angle, Brewster’s angle, at which p­polarized light can
be made to disappear when it reflects off of a surface, leaving only s­polarized light.
This property of reflected light is very useful for generating polarized light; if an
unpolarized beam of light is made to reflect off a surface sitting at Brewster's angle, the
resulting reflected light will be linearly polarized. Specifically, it will be s­polarized
light. This method is sometimes used to ensure that the output beam of a laser is
linearly polarized. This property is also the reason why polarized sunglasses work
well. When you go outside and see a lot of glare, that usually comes from sunlight
reflecting off of buildings, roadways, and cars. This reflected light will be mostly s­
polarized. Polarized sunglasses are designed to block this s­polarized light, leaving only
p­polarized light. This reduces glare and makes everything easier to look at. In this lab,
you will be conducting a series of experiments that will allow you to observe and
determine Brewster's angle for a glass prism.
Procedure
You may enter your data in the tables provided below. You will do a series of
experiments to try to see the difference between reflected light when it is horizontally
polarized vs. vertically polarized. The experimental apparatus should look like Figure
4. The linear polarizer is there to make sure the polarization is vertical or horizontal, as
needed.
linear
polarizer
glass prism
HeNe laser
rotation
stage
Page 31 of 77
power meter
Figure 4 – Experimental set­up for reflectance measurements as a function of
angle of incidence.
Part I.A. S­Polarization (perpendicular) Reflection as a Function of Incident Angle
1. The light incident on the prism is initially set to be vertically polarized. If the
rotation stage is set at 0 degrees, the laser beam should be retroreflecting (going back
exactly the way it came). If it is not, fix it at this time.
2. Rotate the rotation stage in 5­degree increments and align the power meter with the
reflected beam each time. Record the power reading for each increment. Take data up
to an angle of incidence of 80 degrees.
Part I.B. P­Polarization (parallel) Reflection as a Function of Incident Angle
1. Rotate the polarizing film by 90 degrees; this rotates the polarization of the laser
beam by 90 degrees. The beam is now horizontally polarized, in the plane of incidence.
2. Repeat step 2 of Part I.A.
Part I.C. Unpolarized Light (simulation) Reflection as a Function of Incident Angle
1. Rotate the polarizing film to an angle of 45 degrees with respect to the horizontal, so
it is half p­ and half s­polarized.
Page 32 of 77
2. Repeat step 2 of Part I.A.
Part II. Reflection as a Function of Polarization Angle at Fixed Angle of Incidence
1. Set the rotation stage of the prism at an angle of 57 degrees. Leave it at this setting
for this part of the experiment.
2. Rotate the polarizing film from 0 to 45 degrees in 5­degree increments and record the
power readings of the reflected beam. You may enter your results in the table below.
Your instructor will have to rig up a way to measure how much the polarizing film is
being rotated; this depends on the equipment available at your school.
After you have taken all of this data, have your instructor switch the glass prism out
and out the acrylic prism in and repeat your measurements. Note any differences and
discuss them. Do you expect the acrylic to perform perform the same way as the glass?
Why or why not? A class discussion should be held after all the data is taken and the
groups have time to analyze it., this can be a two­week project, with the glass prism
being used one week, the acrylic started the first week and then finished the second
week, and a discussion of the results the second week. Again, the purpose of using the
two different prisms is not only to see the difference (if any) in resulted, but to get
more practice at alignment, data collection, data analysis, and discussion of results.
Being able to draw conclusions from your results is as important as being able to
obtain them is!
Results
Your experimental results should look like the results shown in Figure 2; the s­
polarized light power should gradually rise with increasing angle of incidence. The p­
polarized light reflected power should go to zero at some angle – this is Brewster’s
angle. Calculate what is should be from the formula above, using 1.5 as the index of
Page 33 of 77
refraction for the glass and 1.0 for air, and compare your calculated value to your
experimental value. What should the data from part C do? Well, since it contains an
equal mix of s­ and p­polarized light, it should look like it would fit in between the
curves of Figure 2. Finally, the data from Part II should show you that s­polarized
light will reflect off of the glass prism when it is set at Brewster’s angle, but p­
polarized light will not. This means your reflected power when the half­wave plate is
set at zero degrees should be high and the reflected power when the half­wave plate is
set at 45 degrees should be close to zero. What happened with the acrylic prism?
Table for Results of Part I (glass, above, acrylic, below):
Page 34 of 77
Glass
Angle of
incidence
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
Power ­
Part I.A.
Power ­
Part I.B.
Power ­
Part I.C.
Power ­
Part I.A.
Power ­
Part I.B.
Power ­
Part I.C.
Acrylic
Angle of
incidence
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
Page 35 of 77
Table for Results of Part II (glass, left, acrylic, right):
Angle of
incidence
5
10
15
20
25
30
35
40
45
Power
Angle of
incidence
5
10
15
20
25
30
35
40
45
Power
Page 36 of 77
Equipment List
From the Edmund Optics America Optics & Optical Instruments Catalog, 2005
www.edmundoptics.com
Catalog Location
Number
Name
Price Quantity
p. 133
G53­027
Rotary Assem. w/hold.
$174.10
(2)
p.120
G56­929
Bench Plate
$60.00
(1)
p.128
G03­649
Post Holders 6.0”
$23.10
(2)
p. 128
G36­499
Mounting Posts 4.0”
$10.00
(2)
From the Daigger Sci­Ed Warehouse Catalog, 2005
Catalog Location
Number
Name
Price Quantity
p. 76
DH15012
Light Meter
$174.00
(1)
Use the above equipment with the prisms from the Prism & Lens Set from Lab
#2. This adds an additional $474.30 to the equipment total, but the rotary holder,
bench plate, posts and post holders are also needed for thee spectroscopy lab.
The instructor will need to acquire some polaroid vertically polarizing slides and
a method to mount and rotate them in measured 5 degree increments (or do the
best you can to simulate this), which should be available already in the HS
science lab.
Page 37 of 77
Laboratory #5: Spectroscopy
What’s Cool About Spectroscopy?
Spectroscopy is the science of breaking multi­colored light down into its
individual, pure component colors (wavelengths) so that you can see exactly
which colors are present in the light you are examining. What’s the big deal
about that? It’s amazing, because there is so much you can do with this skill. This
is how forensic scientists identify unknown substances at crime scenes; this is
how astronomers can tell us what gases are contained in the atmosphere of Mars
(or any other planet); this is how scientists can identify the chemicals present in
any unknown sample given to them, if they have spectroscopy available to them.
This is such a powerful tool, it has allowed us to learn about the nature of the
universe, how the universe was formed, how the planets were formed, what they
are made of, and from this information, scientists can figure out other
information, like why the planets contain certain elements from the periodic
table, and why they don’t contain others.
Background
The reason this works is because each element in the periodic table of the
elements has a unique spectroscopic signature. That’s a fancy way of saying that
each element has its own unique finger print, and that finger print just happens
to be its spectrum, which is what the spectrometer reveals to us. Just like every
human being can be identified by either their fingerprints or their DNA, each
element in the periodic table can be identified by its spectroscopic output, which
is the unique pattern of colored bands that the chemical emits when submitted to
certain conditions. The electrons in the sample must be excited by some energy
source, which causes them to absorb energy and raise to higher energy levels,
and then, eventually, they fall back down to the energy levels where they came
Page 38 of 77
from. In so doing, they emit the energy they absorbed, and we see this energy as
light of a specific and unique wavelength corresponding to the energy difference
between where the electron was when it was excited and where it ended up after
it gave up the energy it absorbed. This energy difference, between where the
electron was when it was excited and where it went to after it emitted the energy
it absorbed, is a unique number, and there is only one number that it can be.
That one number, an amount of energy, corresponds to one unique pure color
and shows up as a colored line in the spectrum of that element. There are several
electronic energy transitions which are “allowed” by nature for each element, so
each element will therefore have more than one line in its spectrum, and some of
the spectral lines fall outside the visible range. Therefore, some elements, to us,
will appear to have many lines and others will appear to have very few, but it
might be because we, as humans, can only view a very small portion of the total
electromagnetic spectrum and therefore we are only viewing a portion of the
spectrum that the element is emitting, but it is enough for us to be able to
identify each element. So, every element in the periodic table has a unique
spectrum and therefore, when that element is present in a sample of something
being analyzed using spectroscopy, its spectrum will show up and you will be
able to identify the element.
That’s what you are going to learn to do in this lab. You will be given 4
sealed tubes of gases which are unknown, and you will build a simple
spectroscope which will allow you to view the spectrum of each tube of gas as it
is excited by being plugged into a socket (there are safety issues here – more
later). The tube will glow and have a distinctive color to it, but the color you see
coming from the tube with your eye alone is actually made up of several pure
colors mixed together, and the spectroscope will separate these pure colors for
you, you will identify their wavelengths and compare them to a chart, and by
this method, you will identify the elements contained in each tube. You will
Page 39 of 77
record the wavelength of each spectral line you view and compare it to the
standard values and see how close your values come to the true values.
From this lab, you will know, in part, how forensic scientists help police
solve crimes, how astronomers discover the nature of the universe, and how
unknown substances can be identified to determine if they are dangerous or not
by a poison control center, for example, or in a medical lab, when you send a
specimen in and the doctors are trying to figure out what is making you sick.
Today’s lab teaches you a very powerful skill which is applied in so many
different fields, from chemistry, biology, medicine, astronomy, to police work
and beyond, that being familiar with it is essential if you would like to become a
laboratory technician in many possible fields you may choose to study.
Procedure
A. Building the Spectroscope
Building a simple spectroscope is not as complicated as it sounds. All you need
are the following; 2 meter sticks, a source to analyze, a holder for one meter stick,
a holder for a transmission diffraction grating, and the transmission diffraction
grating. You set them up as follows.
1. Using the meter stick optical bench. Place this along the end of your lab
work area. Behind the 50 cm mark on the meter stick, place the spectrum
tube power supply so that when the gas tube is in the supply, it will be
lined up exactly with the 50 cm mark on the meter stick optical bench. The
meter stick should be lower than the center of the gas tube.
2. Instructor should do this: Use the other, regular meter stick to measure,
from the base of the power supply for the tube of gas, a distance of 1
Page 40 of 77
meter, and place the diffraction grating, mounted in the rotary assembly
and secured so that it is not damaged or able to move during the
experiment, exactly at this 1 meter mark. It is the grating itself that should
be 1 meter away from the base of the supply, not the bench plate, the
rotary assembly, or anything else, so make sure that the distance from the
base of the supply to the actual grating is 1 meter, or 100 cm. Important:
Do NOT ever touch the surface of the diffraction grating. It is expensive
and fingerprints cannot easily be cleaned off of it, and they may
permanently ruin it. If anyone needs to handle it, it should be the
instructor, and then, the grating should only be handled by its edges,
never by touching the flat, ruled part of it. Touching anything but the thin
edges of the grating can permanently damage it, so allow your instructor
to set up the grating and then do not touch it. If there is any problem
during the experiment, ask the instructor to check on it for you. Do not
attempt to make any adjustments to the diffraction grating yourself.
B. Using the Spectroscope
3. Make sure the power supply is plugged in, but turned off. Insert a tube of
gas by carefully pushing down on the spring in the bottom portion of the
holder, then sliding the tube vertically in so that it is under the top spring,
and slowly releasing the bottom spring until the top spring in the top
holder catches the top of the tube. Keep guiding the tube until it will not
go any higher. It is now secured between two conducting springs which
will supply 110 volts to the supply when the switch is turned on.
Important: This power supply should be thought of as a large wall socket;
if it is touched when the power is on, it is the same as sticking your finger
into a wall socket. NEVER touch the tube or the spring holders while the
power is on. Also, the tubes get very hot while working, so when it is
time to switch to another tube, turn the power off, use a piece of paper
Page 41 of 77
towel or paper folded many times over, or some kind of cloth with which
to gently grab the fat portion of the glass tube, NOT the metal conducting
part because that will be even hotter, and not the skinny part because it is
too delicate, gently push down on the bottom spring until the top of the
tube is free from the top spring, pull the tube out, and replace it in its
holder, being very careful not to break the skinny portion of the tube.
Then, place a new tube of gas in using the same procedure outlined
above. It is best to allow the instructor to perform this part of the lab, but
if the students do it, just make sure you do not touch a tube of gas that
has just been running with your bare hands or a single­ply sheet of
paper or paper towel; it is very hot. Waiting a few minutes in between
and doing some calculations is also a good idea, since, once you take the
data, you must calculate the wavelengths of the lines you saw. So, do
some calculating before you switch out tubes, but still assume they are hot
and fragile.
4. Once a tube is loaded in the supply, turn it on and turn off the overhead
lights. Make sure there is some backlight somewhere in the room so
people may move safely about, but before you turn the lights out, make
sure the aisles are clear of obstructions and nothing is in danger of being
knocked off of a lab bench. (For more tips on safe lab practices, see
reference number 5, How to Study for Success in Science, Math, and
Engineering Courses, Amy E. Bieber.) Be careful while working in the dark;
be extra­alert to your movements and where things are. Look through the
transmission grating at the glowing tube of gas, and you should see
colored lines, identical on the left and right side of the light source. These
colored lines should look like they are floating above the meter stick.
Write down every color you see and the number on the meter stick where
Page 42 of 77
you see it, and write it down for the left and the right side for each color.
You are now recording the spectrum of this gas tube.
5. Repeat the above procedure for all 4 tubes of gas, but follow the safety
precautions and do the calculations in between, as already stated.
6. You will now calculate the wavelengths of the spectrum for each tube of
gas as follows. Fill in the following tables:
Tube #1:
Left
Right
position
position of
of line (S) line – 50
cm, then
converted
to meters
Average
of left and
right
positions
(Average
S) in
meters l=
S
2 2 g X + S (see below)
Standard
value of
l in
meters
% error in
your
measurement
(see below)
Standard
value of
l in
meters
% error in
your
measurement
(see below)
in meters
Tube #2:
Left
position
of line (S)
Right
position of
line – 50
cm, then
converted
to meters
Average
of left
and right
positions
(Average
S) in
meters l=
S
g X 2 + S 2 (see below)
in meters
Page 43 of 77
Tube #3:
Left
position
of line (S)
Right
position of
line – 50
cm, then
converted
to meters
Average
of left
and right
positions
(Average
S) in
meters Right
position of
line – 50
cm, then
converted
to meters
Average
of left
and right
positions
(Average
S) in
meters l=
S
g X 2 + S 2 (see below)
Standard
value of
l in
meters
% error in
your
measurement
(see below)
Standard
value of
l in
meters
% error in
your
measurement
(see below)
in meters
Tube #4:
Left
position
of line (S)
l=
S
2 2 g X + S (see below)
in meters
7. The values you read off of the meter stick on the left and right side are
both “S” in the equation for calculating the wavelength. The only
difference is, since the light source is located at the 50 cm mark, you must
subtract 50 cm from the right­side readings sin order for them to be
correct. Once that is done, the left and right side readings for S should
ideally be equal, or in reality, very close. You will average them and use
the average value for your calculation to reduce the error in your answer.
Page 44 of 77
Convert your “S” values to meters by dividing by 100 before entering the
data into the table.
8. Calculate l by using the formula in the table. X is 1 meter; g is the number
of grooves per meter on your grating, which in this case means g = 11000
grooves/meter. When plugging the numbers into your calculator, make
sure you use parentheses to tell the calculator to use the proper order of
operations. Your instructor will explain this. It means, if you don’t use
parentheses, your calculator, just for the denominator, will think you
want “g* X 2 + S 2 ” instead of having the “S2” part under the square­root
sign too, so you have to put parentheses around everything that goes
under the square root sign, and parentheses around everything that goes
in the denominator. If you have trouble with the calculation, your
instructor will help you, and there will be enough spectral lines so that
you will get plenty of practice so by the end of the lab, you will know
exactly how to use that equation, easily!
9. Compare your value of l in meters to the standard value supplied in the
table. Calculate the % error as follows: % error =
exp erimental - s tan dard s tan dard * 100 % 10. The “bars” are absolute value signs; they mean that if your answer comes
out with a minus sign in front, throw it away. We don’t care if the
experimental answer was too big or too small, we only care by how much
it was too bog or small. Anything less than 20% is considered good for an
experiment using this sort of equipment.
Page 45 of 77
11. Repeat the calculations for all experimental lines you saw. Be extra careful
to look for the violet and blue lines; they can be very faint and hard to see,
so if you appear to be missing some lines, go back and look for the violet
and blue ones. Also, in one case, there might be some yellow lines too
close together to read separately, so consider them one line for calculation
purposes, but when trying to identify the spectrum by comparing it to the
chart, remember that there was a pair of yellow lines. Write it down
somewhere.
12. Once you have all of your wavelengths computed, compare your results
to the spectrum of gases chart and try to identify which gas was in which
tube. They are color­coded and your instructor will have the answers.
Note that the color code on the tube gives no hint as to anything to do
with the color or name of the gas inside!
Equipment List
From the Daigger Sci­Ed Warehouse Catalog, 2005
www.daigger.com, 1­800­621­7193
Catalog Location
Number
Name
Price Quantity
p. 75
CH3531
Spect. Tube Pwr. Sply.
$161.00
(1)
p. 75
DH3531G
Helium Gas Tube
$33.00
(1)
p. 75
DH3531M
Neon Gas Tube
$33.00
(1)
p. 75
DH3531H
Hydrogen Gas Tube
$33.00
(1)
p. 75
DH3531L
Mercury Gas Tube
$40.00
(1)
Use this with the meter stick optical bench from lab #3 and the diffraction
grating listed below. Another meter stick will need to be supplied by the
instructor, along with a base of some sort (a piece of wood will do just fine) to
Page 46 of 77
elevate the gas tube to the correct height to match the height of the meter stick
optical bench. Also, use the bench plate, post holder, post, and rotary assembly
with holder from the polarization lab to mount the diffraction grating. It does
not need to rotate, but it needs to be securely fastened so that it will be stable and
will not move once initially set up. Only the instructor should handle the
diffraction grating and the students should be admonished not to touch it once it
is set up, and if any problems occur, the instructor should fix it by touching the
edges of the grating only; NEVER the middle, flat, ruled portion.
From the Edmund Optics America Optics & Optical Instruments Catalog, 2005
www.edmundoptics.com
Catalog Location
Number
Name
Price Quantity
p. 114
G46­073
Transmission Grating
$123.40
(1)
This grating has 110 grooves/mm, which means the spacing between grating
lines is 9.09 microns, or 9.09 x 10 –6 meters per groove and there are 11,000
grooves per meter (g in the formula). This adds an additional $423.40 to the
equipment total.
Page 47 of 77
Laboratory #5B: Monochromators (if you have one)
What’s Cool About Monochromators?
The color of visible light depends on its wavelength. Reds and oranges
have the longest wavelengths of visible light while blues and violets have the
shortest wavelengths. But light comes with different levels of purity of color. It
can be like laser light, containing a very narrow range of wavelengths, it can be
like a neon sign, containing a larger but still limited number of different
wavelengths, or it can be like an incandescent light bulb, which contains every
color of the rainbow. How do you tell? If you need a laser beam to be extremely
“spectrally pure,” meaning that it contains very few different wavelengths, how
do you measure that? You can use a monochromator. It can tell you if your light
source has the spectral purity you need. Lasers usually come with a specification
sheet that tells you the spectral purity of the beam; someone has to do those
measurements, which means that someone working at the company building the
lasers has to know how to use a monochromator. Researchers also use them to
test the lasers they already have, so if you want to work with a scientist in a
research lab, it would be helpful to know how to use a monochromator.
Unfortunately, monochromaters are pretty expensive. The ones available at
Queensborough are fairly simple quarter­meter monochromaters, and they cost
about $6,000 each in 2005. But if you have them available, they are very useful
tools. This lab will show you how to use one, and what the results from it mean.
Background
In this exercise you will learn to use a monochromator to determine the
wavelength of several different colors of light emitted by the helium­neon laser.
The helium­neon, or HeNe laser, is the laser used in many supermarket scanners,
surveying equipment, and some laser pointers. It most commonly emits red
light, but the ones you will use today are tunable HeNe’s and are capable of
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producing red, orange, yellow, and (in some cases) green light. The
monochromator is a device which measures the wavelength content of light
entering it. All light, including laser light, consists of a spread of wavelength
values rather than just a single wavelength value. Most ordinary light sources,
like lightbulbs, emit many wavelengths across the entire visible spectrum. Lasers
emit relatively pure light, containing a small range of wavelength values. The
monochromator can measure the wavelength spread for each light beam
entering it. Figure 1 shows some of what is inside the monochromator.
Curved
mirror
Entrance
slit
Diffraction grating
(rotatable)
Exit slit
Figure 1. Basics of a monochromater.
The grating in Figure 1 separates the incoming light according to wavelength
(color); as the light travels away from the grating, the colors spread out in space,
so that when they get to the exit slit, only one wavelength will pass through; the
others will be blocked. When you turn the knob on the monochromator, you are
rotating the grating so that different wavelengths become aligned with the exit
slit and come out of the monochromator. In this way, you can read the dial by
the knob and know exactly what wavelength of light is exiting the
monochromator. Actually, the monochromater is a little more complex than the
diagram, but the diagram will give you a good idea about what happens inside.
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The wavelength spread contained within the laser beam will not contain
equal amounts of power at all wavelengths; the power distribution will look
something like that shown in Figure 2.
maximum
power
half­maximum
points
wavelength
Figure 2. Power as a function of wavelength for a typical laser beam.
Notice that the shape of the power curve has some labeled features on it. The
point where the power reaches its highest value is the maximum. The maximum
occurs at the center wavelength, which ideally should be the wavelength right in
the middle of the spread of wavelengths contained in the power curve. You will
look for the power maximum and record the center wavelength at which the
maximum occurs as part of this experiment.
Notice the two arrows that point to the half­maximum points. There are
two points on either side of the maximum at which the power drops to half of its
maximum value; these are called the half­maximum points. Record the power
and wavelength at these half­maximum points. Then, subtract the two
wavelength values of the half­maximum points to obtain the full­width at half­
maximum (FWHM) for the laser beam. The FWHM is a measure of the
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wavelength width of the laser beam, measured between the half­maximum
power points.
Procedure
1. The monochromator comes with several small slits which can be inserted at
the entrance and exit apertures of the device ­ begin by removing these slits.
**Important: the dial on the monochromator that selects which wavelength
will pass through should not be forced; if it stops turning in one direction, do
not try to force it.**
2. Begin with red laser light ­ this should be at a wavelength of 632.8 nm (nm
stands for nanometer ­ one billionth of a meter). Set the dial on top of the
monochromator for 632.8 so that it will allow light of this wavelength to pass
through it. Align the monochromator with the laser so that the red laser light is
going into the monochromator and coming out the other side. Use a white piece
of paper to see if the laser beam is coming out of the monochromator – DO NOT
LOOK INTO THE MONOCHROMATOR WITH YOUR EYES!
3. Align the detector with the output laser light from the monochromator. You
should see a power reading on the detector scale. If you don’t get a reading, try
adjusting the scale on the detector until you get a reading. If it says “1.” then it is
overloaded and you need to turn to a less sensitive scale.
4. Slowly adjust the wavelength knob on the monochromator until the output
power is at its maximum value. Write down the wavelength at which this
happens in Table 1 on the following page. Also, make a note of the maximum
power level.
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5. Turn the wavelength knob so that the wavelength increases until the power
on your detector drops to half its maximum value ­ record this wavelength in
Table 1.
6. Now turn the wavelength knob so that the wavelength decreases until the
power on your detector drops to half its original maximum value ­ record this
wavelength in Table 1.
7. The difference between the wavelengths you calculated in steps 5 and 6 is
called the Full Width at Half Maximum (FWHM) of the laser wavelength. What
is it? Write it in Table 1.
8. Insert the 150 um slits into the entrance and exit apertures of the
monochromator and repeat the above exercise, using Table 2 for your results.
Did the FWHM and center wavelengths change?
9. Now switch the laser to orange (yellow, green) light and repeat the above
procedure – the only difference now is, you have to determine the wavelength of
the orange light. When you first switch the laser over, nothing will come out of
the monochromator because the dial is set for red light. Gradually decrease the
wavelength on the dial until orange light begins to appear on the exit side of the
monochromator. Record the wavelength for maximum power, and then repeat
the rest of the experiment. You can fill in the tables below with all your data.
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Table 1 – Experimental data for monochromator using no slits.
Color
center wavelength
two wavelengths
FWHM , no slits
at
which
power
is
(wavelength where
(difference
half maximum
power is maximum)
between the
and max power, no
2wavelengths
slits
where power is
half maximum)
red
wavelength:
power:
orange
wavelength:
power:
yellow
wavelength:
power:
green
wavelength:
power:
Table 2 – Experimental data for monochromator using 150 mm slits.
color
center wavelength two wavelengths
FWHM , 150 mm
at which power is
(wavelength
slits (difference
half maximum
where power is
between the
maximum) and
2wavelengths
max power, 150
where power is
mm slits
half maximum)
red
wavelength:
power:
orange
wavelength:
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power:
yellow
wavelength:
power:
green
wavelength:
power:
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Laboratory #6: Analog Oscilloscopes What’s Cool About Analog Oscilloscopes? An oscilloscope displays voltage wavefor ms (electric signals) as a function of time or frequency (nu mber of cycles per second). Almost any career involving electronics, lasers, audio / visual, or computer engineering requires the use of an oscilloscope. Recording studio engineers, auto mechanics, and air conditioning / refrigeration repair technicians use oscilloscopes because all electrical and mechanical equip ment is becoming more sophisticated and uses more high­tech circuitry than it used to. Oscilloscopes are so common in technology that they are usually just called “scopes.” Analog scopes are the older and easier to use variety that you will use first. If there is time, we will use more advanced digital scopes too. Background
An oscilloscope is an instrument that displays voltage waveforms as a
function of time or frequency; the voltage level is displayed on the vertical, or
“y” axis, and the time or frequency is displayed on the horizontal, or “x” axis.
Scientists and technicians in optics, electronics, biology, and mechanical
engineering, just to name a few, use oscilloscopes. Auto mechanics and people
who repair electrical appliances also use them. In this lab, you will operate an
analog oscilloscope using a function generator that will provide a repeating
voltage signal to the scope. In an analog scope, a beam of electrons hits the
phosphor­coated screen of the tube that we view. This causes the phosphor to
glow for a brief time. The strength of the signal controls the vertical position of
the glowing spot by moving the electron beam up and down. Usually a timer
inside the scope moves the beam back and forth horizontally across the screen.
This combination of motions lets us see the signal strength as a function of time.
If the signal is a repeating one, we get a graph of the signal versus time holding
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steady on the screen. This allows us to measure characteristics of the repeating
signal. (A digital scope works differently, but displays basically the same
information.)
Procedure
In the lab, you will be given a sheet summarizing the different knobs and
buttons on the oscilloscope. Your instructor will give an overview of how the
scope works and will explain how you can test the function of each control on
the scope. You will learn how the scope acquires and displays a voltage
waveform that is steady on the screen (“triggering”), how to set the vertical scale
to display the portion of the waveform you want to view (“volts per division”),
how to set the horizontal scale to control how many cycles of the periodic wave
you will display (“time per division”), how to read the display, how to position
the waveform (vertical and horizontal position knobs), and many other
functions. After you become familiar with the scope, the instructor will “mess
up” the settings on your scope to see if you can get the display back the way it
was originally. This will test your understanding of the controls on the analog
oscilloscope.
By the time the lab is over, you should be able to display a signal from the
signal generator, measure its maximum voltage, and measure the time for one
repetition of the repeating pattern (the period), Based on your measurements,
you should be able to calculate the frequency of the signal.
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Laboratory #7: Prisms
What’s Cool About Prisms?
Prisms are interesting optical elements because they disperse, or spread
out, the components of multi­colored light that enter them. Prisms can be used in
spectrometers instead of the grating that you used in experiment number 5, so
they have all the power that spectroscopy has because they can be used to build
a spectrometer. They can also make rainbows (more later)!
Background
Light entering a prism is refracted (recall lab #2) twice – once upon
entering at some angle other than 0 or 90 degrees, and again upon exiting the
prism at some angle other than 0 or 90 degrees. This double refraction is what
causes the light the spread out. Recall that refraction is wavelength­dependent,
so the amount that the light beam will be bent upon entering and leaving the
prism will depend on its color (wavelength), and therefore different colors will
bend by different amounts, and when the light emerges from the prism, the
component colors of that light will be spread out in space, like a rainbow. A
prism can be used to create a rainbow, and real rainbows in the sky are formed
because of rain droplets, which simulate the shape of a prism, or, you could say,
the prism simulates the shape of the rain droplet! (See reference #4, Is There a
Laser in the House? Understanding Your High­Tech Everyday World, Amy E. Bieber,
for a more complete discussion of rainbows and dispersion.) The amount of
“bending” that the light undergoes in the prism is called deviation, and there is
an angle of incidence that will lead to a minimum deviation angle for the output
of the prism. In this lab, you will determine the minimum deviation angle for the
two prisms provided in the Prism and Lens Set – one glass, and one acrylic.
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The minimum deviation angle for the prism is given by the following
formula:
d = q i1 + q t 2 - a
where
d = deviation angle in degrees
qi1 = angle of incidence of light compared to surface normal on left side of prism
qt2 = angle of exit of light compared to surface normal on right side of prism
a = apex angle of prism, which the instructor will tell you (usually 60 degrees,
but ask to make sure)
The above quantities are illustrated in the following diagram:
surface
normals are
dotted lines
a
qi1
incident
beam
qt2
exiting
beam
Figure 1 – Illustration of quantities needed to calculation minimum deviation
angle for a prism.
Procedure
1. Place the prism on the paper on top of the corkboard and trace its outline,
just like you did at the beginning of lab #2.
2. The laser should be positioned by the instructor on the table so that it is at
the correct height to pass through the prism. Angle the laser beam so that
it strikes the prism surface at some oblique angle, like that shown above.
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Make sure that the laser beam is exiting the prism on the right side, and
not the bottom, top, or same side it came in! Also remember to be careful
about where stray laser reflections go, and block the path of any laser
beam that is reflecting off of something and leaving your work area. Mark
the path of the laser with two pins, both entering and exiting the prism.
3. Remove the pins and prism and construct the drawing shown in figure
#1. Calculate the deviation angle.
4. Chances are that you did not come upon the minimum deviation angle
the first time you tried, so there isn’t much chance that your calculated
value is the e minimum deviation angle for this prism. So, get another
piece of paper, repeat the set­up process, and this time, angle the laser
beam while watching the exiting beam on the right side of the prism.
Angle the laser until you think, by eye, you have found the minimum
deviation angle, which means that the laser beam is emerging from the
prism at the smallest possible angle compared to the surface normal,
which you may draw on the paper to help you, but since the laser beam
will be moving around, it will not always be exactly coincident with
where you drew this normal, but you can still, by eye, compare the
position of the exiting laser beam with this normal and decide if the angle
has reached its minimum.
5. When you think you have found the angle of minimum deviation, mark
the positions of the beams with the pins again, remove the prism, measure
the angles, do the calculation, and ask the instructor if your answer is
correct. I’ll give you a hint; if it turned out that the two angles theta (input
and output) that you had to measure were equal, and if it turned out that
the line traced inside the prism, denoting the laser beam’s path through
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the prism, is parallel to the bottom base of the prism, then you have found
the minimum deviation angle, or something very close to it.
6. Repeat this experiment for the other prism (one is glass, the other acrylic,
so they will have different indices of refraction and therefore your
answers should be different, but not hugely different).
After all the data is taken and everyone has successfully found the
minimum deviation angle, a class discussion should ensue as to why the
entrance and exit angles had to be equal for the deviation angle to be minimum,
and why the laser beam should travel parallel to the base of the prism when it is
following the path of minimum deviation. Think about this, use common sense,
and you should have and interesting and lively discussion with a lot of original
ideas being floated about. Also, why was there no rainbow? Because you were
using laser light, which is pure in color, and therefore the prism cannot spread
the beam out any farther than it already is. Had you used white light, you would
have gotten a rainbow, and since the optical meter stick bench kit comes with a
flashlight, you could try to use this flashlight with the prisms to create a
rainbow.
Equipment List
From the Daigger Sci­Ed Warehouse Catalog, 2005
www.daigger.com, 1­800­621­7193
Catalog Location
Number
Name
Price Quantity
p. 76
DH9283A
Prism & Lens Set, Glass
$61.00
(1)
p. 76
DH9283B
Prism & Lens Set, Acryl. $51.00
(1)
Also needed is the laser and the corkboard and pins used in the refraction lab,
and the flashlight from the optical meter stick bench kit. This adds an additional
$112 to the total cost.
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Laboratory #8: Beam Expanders and Spatial Filters
What’s Cool About Beam Expanders and Spatial Filters? Beam expanders make laser beams bigger and spatial filters make then “cleaner,” meaning more unifor m in intensity. After a laser beam has been expanded and spatially filtered, the edges will be as bright as the center, which wo uld otherwise not be true. This is important because wit hout an expanded, spatially filtered laser beam, holography would be impossible. Spatial filtering and beam expansion is also used in defense / military applications like optical pattern recognition. It can be used, for example, to take a high­altitude photo of a tank on the ground and then deter mine if it is a friendly or enemy tank. The same is true of any other piece of large equipment on the ground that might be p hotographed fro m a plane or satellite and that needs to be ID’d by very fie features not detectable to the hu man eye. So, if you work in t he aerospace or defense industry, yo u might find yourself using a spatial filter / beam expander. Holography of course would be impossible witho ut this device, but that will be discussed in a fut ure lab.
Background
In this exercise you will learn how to use lenses in combination to make
two very important optical components – a beam expander and a spatial filter. A
beam expander uses two lenses to increase the diameter of a laser beam; a
spatial filter does the same thing, but it also “cleans up” the beam and makes it
extremely uniform in intensity across the entire diameter of the beam. This is
important for holography, which is the lab we will do next week with the use of
the spatial filters you will set up this week. Beam expanders are important for
many applications that involve magnification, like building telescopes and
microscopes.
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A laser beam is usually small enough that you do not notice that the
center of it is brighter than the edges. When the beam is made larger (using a
beam expander), the fact that the edges are dimmer than the center becomes
much more noticeable. When you want to make a hologram, you need the laser
beam to completely light up the object and you need the edges of the beam that
does this to be just as bright as the center of the beam. In order to make this
happen, you will use a spatial filter. Let’s look at how beam expanders and
spatial filters work.
Beam Expander
A beam expander simply is made of two converging lenses separated by
the sum of their focal lengths. If you recall the lab on combinations of lenses, this
is the same way we made a simple telescope. A beam expander basically does
the same thing as a telescope; it has parallel input and output rays and it
provides magnification. It looks like this:
incoming
small
beam
outgoing
large
beam
lenses separated
by the sum of their
focal lengths
Figure 1 ­ Beam Expander
Spatial Filter
A spatial filter is a beam expander with one additional element: a pinhole
aperture. The pinhole aperture is placed at the point where the two focal
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distances of the lenses meet (as shown below) and its purpose is to cut off the
tails of the laser beam as it passes through. This way, when the laser beam is
expanded, it will have uniform brightness across its entire diameter because the
dim parts that were there before got filtered out by the pinhole.
incoming
small
beam
outgoing
filtered large
beam
pinhole
aperture
Figure 2 ­ Spatial Filter
The reason this works is because before the laser beam gets filtered, its intensity
looks something like this:
center of
beam
distance
Figure 3 ­ Laser Beam Intensity Profile
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The pinhole aperture only allows the center part of the beam to get through, so
then when it gets expanded, it will not be dimmer at the edges:
expand only
this part
pinhole only
allows this
much
through
distance
Figure 4 ­ Spatial filter cuts out all but the central part of the laser beam.
Procedure – Beam Expander
1. Look at the two lenses you are given – their focal lengths are 15 cm and 40
cm. Determine the separation you need to build the beam expander, and do
it on the optical rail using the lens holders. Adjust the lens positions until
you have a collimated beam coming out of the beam expander; collimated
means it remains the same size as it travels away from the laser.
2. Use a ruler to estimate the size of the input beam and the output beam –
determine the magnification of the beam expander by dividing the output
beam size by the input beam size. Describe the quality of the beam – is it
uniform, or is it brighter in the middle than it is on the edges?
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Procedure – Spatial Filter
1. The spatial filter assembly is made up of the collimated lens set­up from
Part One with the addition of the pinhole inserted exactly at the spot
where the laser beam is its smallest. Determine where that spot is, insert
the pinhole into the card holder from the meter stick optical bench kit, and
try to get the laser beam to pass through the pinhole when the laser beam
is its smallest. A little bit of the laser beam’s edge should be blocked, or
“chopped off” by the pinhole – that’s the whole point. This weaker edge
of the beam is what we are trying to eliminate in order to “clean up” the
beam profile and make it have a more uniform intensity across its entire
diameter when it becomes expanded.
2. Adjust the vertical and horizontal position of the pinhole until the light
coming through is maximized. Look at the quality of the expanded light.
Is it uniform in intensity across its diameter? If not, one of two things is
wrong. Either your pinhole is not the right size, or your pinhole is in the
wrong place.
3. The easier thing to check first is whether the pinhole is in the right place.
So, repeat the step above, adjusting the vertical and horizontal position of
the pinhole, also slightly adjust the positions of the lenses if you think the
beam is not perfectly collimated, and again check the beam quality.
Remember, these three components are working together to create a
uniform beam, so they might all three need to be adjusted, and this is an
iterative process, meaning you must do the same thing over and over a
few times before you get it right, like back in lab one with the alignment
procedure. If the beam is reasonably uniform, then you have built your
spatial filter. If it is not, then you have one more step to add and then
repeat the above process.
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4. At its best position, look at the laser beam striking the pinhole and
determine if you think too much of the beam is being chopped off as it
passes through, or if not enough of it is being chopped off. If too much is
being chopped off, your enlarged laser spot will be dim and will fade a lot
at the edges. If not enough of the laser spot is being chopped off, your
enlarged beam will look much brighter in the middle than at the edges
and it will not look “clean.” By looking at the way the laser spot is striking
the pinhole and the way the output beam looks, determine if the pinhole
needs to be enlarged or made smaller, remove the pinhole, either enlarge
it or make it smaller (the pinhole is created by wrapping tape around the
slit from the Optics Set, so to enlarge or shrink the pinhole, you simply
need to add or remove some tape), and re­insert it.
5. Repeat steps 1 ­ 4 until you have an intense, round, uniform laser beam
emerging.**This takes time and patience – don’t expect this to happen in 5
minutes!
****IMPORTANT – remember to practice good laser safety habits when
removing and inserting components into the path of the laser beam.****
Equipment List
This experiment can be constructed using the parts from Lab #3 (the meter stick
optical bench kit and lenses) and lab #1 (the slits) and the following, which can
also be used in lab #7:
From the Edmund Optics America Optics & Optical Instruments Catalog, 2005
www.edmundoptics.com
Catalog Location
Number
Name
Price Quantity
p.26
G45­298
dbl.convex 40cm fl lens
$33.80
This adds an additional $33.80 to the equipment total.
(1)
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Laboratory #9: Holography
What’s Cool About Holography?
Everything! Most people have never even seen a high­quality hologram,
but in this lab, you will be making them. Holography uses a laser to make an
incredibly realistic three­dimensional image of an object, but there’s more to it
than that. There are actually high­tech uses for holography, not just artistic ones!
Holography can be use for three­dimensional data storage. The CDs and disks
you use in computers now to store data are two­dimensional; they store data on
only one surface. A holographic data storage system can store data in multiple
layers, in three dimensions, and so can dramatically increase the amount of data
you can store on a CD­sized disk. Research is currently being done on this, but it
might someday be in every home. Understanding holography now could help a
future scientist or technician if holographic data storage ever gets off the ground.
There are other industrial uses of holography. For example, one of the methods
for checking whether the propellers inside big jet engines are perfectly made
uses a technique called double image holography. Besides, it’s a lot of fun and
really interesting to make a holographic image and to understand what makes it
special. This lab will introduce you to these things.
Background
Holography is the process of using a laser to record the information
needed to reconstruct a three­dimensional image of an object on a holographic
plate. What this really means is that the hologram reconstructs the exact pattern
of light that came from the original scene. Keep in mind that when we “see” an
object or a scene, we do not really see the thing itself. Instead, we see the pattern
of light that comes form the object or scene. A hologram allows us to reproduce
the same pattern of light. Therefore, when we view the light from the hologram,
we “see” the same scene that led to the hologram. In order to understand how
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this works, we need to first discuss the idea of interference, which is what
happens when two or more light waves meet at the same point in space. Picture
light waves as sine waves that travel through space, like this:
Electric field
strength
distance
Figure 1 – An example of a light wave.
Let’s say that two such waves meet at some point in space. What happens to
them? They simply add together. If you have two waves that meet, you add
their field strengths, point­by­point, at each point in space where they meet to
form what is called a resultant wave. Now, this resultant wave can be larger than
either of the two separate waves, or it can be smaller than either of them. It all
depends on the relative phases of the two waves when they meet. Phase refers
to which part of the wave we are looking at in a particular point in space; we
usually speak of the phase of one wave compared to another. For example, the
two waves shown in Figure 2 have a phase difference of 180 degrees:
Figure 2 ­ Two waves that have a phase difference of 180 degrees.
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As you can see, a 180 degree phase difference means that the waves are exact
opposites of one another: the crests of one line up with the valleys of the other.
This is called “180 degrees out of phase.” This means that if we were to add them
together, they would completely cancel each other out. This is called destructive
interference.
Now, if two waves have no phase difference between them, we say that
they are “in phase,” which means that the crests and valleys of the two waves
line up perfectly. If we add two in­phase waves together, we get a resultant
wave that is larger than either of the two separate waves; this is called
constructive interference. If the two waves line up in any way other than exactly
in phase or 180 degrees “out of phase,” we get a more complicated wave pattern,
but it is still called interference.
The next thing you need to know to understand holograms is that laser
light is coherent. This is what makes it so powerful and dangerous! Coherence
means that all of the light waves in a laser beam travel together with the same
phase, meaning that they have constructive interference and none of the power
gets wasted. In an ordinary light bulb, the waves do not travel together with the
same phase; all the light waves from a light bulb have different phases and they
vary randomly, which means that many of them have destructive interference
and power is wasted. This is part of the reason that a few milliwatts from a laser
can blind you but a 60 watt light bulb can be looked at briefly without irritating
your eyes.
Because a laser beam is coherent, it can tell how far it has traveled – that is
something unique to laser light and it is what makes holograms, CD players, and
DVD players work, to name a few examples. A laser beam can tell how far it has
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traveled because the phase does not randomly vary, which means that we can
split the laser beam in two and keep one beam as a “reference beam” and use the
other to send out to bounce off of an object and come back (the “object beam”).
When the object beam comes back, we compare it to the reference beam, and the
difference in phase between the object and reference beams will tell us exactly
how far the object beam has traveled.
Why? Because phase difference is related to wavelength, and we know the
wavelength of the light we are using. Wavelength is the physical length of one
cycle of a wave (the length of the smallest repeatable unit of the wave); Figure 2
shows two cycles, or two wavelengths, of each wave. For example, look at
Figure 2 – can you tell that the top wave is just like the bottom wave shifted by
half the wavelength of the wave? That’s what it means when two waves are 180
degrees out of phase with one another – it means that one of them is shifted a
half­wavelength compared to the other one.
Let’s look at the following example: 1) start with two waves that are in
phase, use one as a reference and use the other to bounce off of an object; 2) after
the waves are recombined, they have a 180­degree phase difference; 3) this
means that the one that went out to the object traveled a half­wavelength more
or less than the reference beam in the same amount of time; 4) therefore, the two
beams, when compared to one another, ended up shifted by a half­wavelength.
So, a 180 degree phase difference corresponds to a half­wavelength in
distance units. Different distances to the object will correspond to different
phase differences between the object and reference beams. For example, a 90­
degree phase difference corresponds to a quarter­wavelength, and 360 degrees
corresponds to a full wavelength. (You should be able to see the relationship
between phase and wavelength from these examples.) These phase differences
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are recorded in an interference pattern on a holographic plate; the interference
pattern is simply the addition of the object and reference waves at each point in
space where they meet. The phase information (which is actually distance
information) is contained within the interference pattern itself because the phase
differences between the waves determine what the interference pattern looks
like. For example, the interference pattern will contain spots of constructive
interference when the object and reference waves are in phase, or destructive
interference when the object and reference waves are 180 degrees out of phase.
For all other phase differences corresponding to different distances to features on
the object (ranging from 0 to 360 degrees), the interference pattern is more
complicated­looking, but it still gets recorded on the holographic plate. So, this
is why a hologram contains both distance (depth) information as well as
brightness information from the object, and therefore looks three­dimensional.
Regular pictures do not look three­dimensional because they only contain the
brightness information and no depth information. This is because a regular
camera looking at light that is not coherent has no way to record phase
information, which is what tells us about distance.
Figure 3 shows a simple set­up for recording a hologram, which is what
you will do in this week’s lab exercise. The reference beam is simply the
expanded and filtered laser beam as it hits the transparent holographic plate; the
object beam is the part of the laser beam that bounces off of the object and is
reflected back towards the holographic plate. When the reference and object
beams meet, an interference pattern is formed that is recorded on the
holographic plate. This plate must then be developed using a method similar,
but not identical, to the method used to develop regular 35 mm camera film. The
details on how to develop the plates will be given in the lab when we make the
holograms.
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reference beam
filtered and
expanded laser
beam
object beam
object
transparent holographic
plate records interference
between reference and
object beams
Figure 3 – Set­up for recording holograms.
To view the holograms that we will make in class, an ordinary white light
bulb can be used, but the holograms will look even better if they are viewed with
the same laser set­up used to record them. In the lab, we will attempt to view the
holograms we make both with a bright filtered light bulb (holding the hologram
in front of the bulb and looking down through the plate to try to see the image)
and with the laser set­up. To view the hologram with the laser set­up, the plate
should be placed back on the stand where it was recorded and you should look
through the plate (away from the laser, not towards it!) to view the image, which
should look very three­dimensional and like it is sitting behind the plate.
Note that when you look at the holographic plate itself, you don’t see the
image; it is not like looking at a photo. The plate only contains the interference
pattern; the image from the plate is seen “floating” behind the plate when it has
light hitting it and you look at the right angle.
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Procedure for Recording the Hologram
1.
Your lab bench will be set up with a filtered, expanded laser beam ready
to use when you arrive. Do not make any adjustments to the spatial filter
assembly.
2.
Select an object of yours to use to make the hologram; things that work
well include keys, coins, and jewelry.
3.
Place the object in the clip provided and set it about two inches behind the
clip which will hold the holographic plate. Set the object at a slight angle
compared to the way the plate will stand. (Your instructor will help with
this.).
4.
Make sure all obstacles in the room are cleared out of the way before the
experiment begins; make sure the laser beam is covered and all lights in the
room are off .
5.
One student form each lab group will come to the front lab bench where
the instructor will give you the holographic plate. You can use the green­
filtered flashlights to see where you are going, but try to avoid shining the
light directly on the plate.
6.
Place the plate in the clip and set it on the stand in front of your object.
The instructor will come around and make sure the plate placement is
correct.
7.
When everyone is ready, the instructor will say, “go,” and you will
unblock the laser beam and expose the holographic plate for 2 – 3 seconds.
The instructor will say, “stop” when it is time to block the laser again.
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8.
Students will then develop the plates using the step­by­step instructions
given in the lab. All but the last step must be done in darkness except for
indirect green light from the flashlights, so be careful about moving around.
Handle the plates with the tongs, by the edges, and try not touch them or
clang them around in the beakers too much.
9.
After development, we will view the holograms and then repeat the
experiment.
NOTE: The Queensborough program uses holographic film plates and chemical
supplies purchased from Integraf LLC . Their website is at www.integraf.com.
Instructions for their materials are supplied also.
Page 74 of 77
Laboratory #10: High­Power Laser Demonstration
What’s Cool About High­Power Lasers? High­po wer lasers are used in many different applicatio ns. In this lab you will see two or three of them operate and you will see the insides of a few others. You will see ho w just a few watts can set a piece of wood instantly on fire, and yo u will learn what makes these lasers so powerful. It is good to know about high­power lasers because peo ple who can design,. build, test, install, and repair them are in demand, and the demand will continue to grow. This lab will introduce yo u to high­po wer lasers so that you can see if a career pat h related to them wo uld interest yo u. These lasers are used in diverse fields such as medicine, manufacturing, academic research, and t he military. People who can work wit h these lasers are needed all over the country.
The Lab
This last laboratory for the lasers and optics portion of the project will
consist of a demonstration of some of the different high­power lasers available in
the QCC Physics Department. The first laser we will demonstrate is a carbon­
dioxide laser capable of emitting approximately 10 watts of laser power. This
laser emits radiation with a wavelength of 10.6 mm, which is in the far infrared
portion of the electromagnetic spectrum; human eyes cannot see infrared
radiation, so the extremely powerful “light” that comes out of this laser will be
invisible to you. It is really essential for you to wear protective goggles when this
laser is on. (In fact, your instructor will only turn the laser on after verifying that
everyone is wearing appropriate goggles.) The beams from the lasers we will be
demonstrating are invisible but they can damage your eyes if you are not
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wearing eye protection, so once these lasers are turned on, do not take off your
goggles for any reason. We will demonstrate this laser cutting very precise and
clean­edged holes in styrofoam. Carbon­dioxide lasers are used in industry for
cutting, drilling, and marking anything from wood to metal. They can make
cleaner and more precise holes and marks than can mechanical drilling
equipment.
The second laser we will demonstrate is called a neodymium­YAG
(Nd:YAG) laser, where YAG stands for yittrium aluminum garnet and is a type
of crystal. If you looked at the inside of this laser where the energy source is, the
Nd:YAG “rod” would look like a very clear, pink, polished piece of glass. This
laser is capable of emitting 40 Watts of continuous power, or 10 Watts (average)
of concentrated pulsed power. Even though the average power for the pulses is
lower than the continuous power, the pulses are very dangerous because they
concentrate that power into very short bursts with high peak powers. This laser
emits at a wavelength of 1.06 mm, which is also in the infrared (invisible) portion
of the spectrum. You will need to wear a different pair of goggles for this laser
demonstration; goggles are designed to protect your eyes from very specific
wavelengths and you can’t use just one pair of goggles for all kinds of different
lasers. The YAG laser will be used to burn a hole in a piece of wood, and you
will see how fast this happens and how clean the edges of the burned area are. A
laser like this is also used for drilling, machining, and for laser artwork! This
laser is designed so that a special crystal can be installed, called a “frequency­
doubling crystal,” which will turn the 1.06 mm light into 532 nm light, which is
green. When you do this, you lose about half (or more) of the power, but it is still
a very powerful beam.
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References
1. “Laser Academy Optics Laboratory Manual,” Queensborough Community
College Physics Department, available at
http://www.qcc.cuny.edu/physics/lfot/LFOT_FAQs.asp, click button on left
side of screen that says “Laser Academy lab Manual.”
2. Daigger Sci­Ed Warehouse Catalog, 2005, www.daigger.com, 1­800­621­7193.
3. Edmund Optics America Optics and Optical Instruments Catalog, 2005,
www.edmundoptics.com, 1­800­363­1992.
4. Is There a Laser in the House? Understanding Your High­Tech Everyday World,
Amy E. Bieber, Pearson Custom Publishing, 2005. Available at
www.pearsoncustom.com. ISBN 0536919038.
5. How to Study for Success in Science, Math, and Engineering Courses, Amy E.
Bieber, Pearson Custom Publishing, 2005. Available at
www.pearsoncustom.com. ISBN 0536943222.
6. Queensborough Community College Astronomy Laboratory Manual, 2003.
Available at QCC Bookstore.
7. Hofstra University Physics Department Astronomy Laboratory Manual, 2000.
Available from Hofstra University Physics and Astronomy Department.
Page 77 of 77
Composite Equipment List and Total Cost
From the Daigger Sci­Ed Warehouse Catalog, 2005
www.daigger.com, 1­800­621­7193
Catalog Location
Number
Name
Price Quantity
p. 75
DH3529A
HeNe Laser, 0.5 mW
$410.00
(1)
p.75
DH3361A
Light Box & Optics Set
$85.09
(2)
p. 76
DH3509
Economy Optics Kit
$24.00
(4)
p. 76
DH9283A
Prism & Lens Set, Glass
$61.00
(1)
p. 76
DH9283B
Prism & Lens Set, Acryl. $51.00
(1)
p. 76
DH3508
Meter Stick Opt. Bench
(2)
p. 76
DH3508A
38 mm double convex lens $5.00
(2)
p. 76
DH3508C
38 mm dbl. concave lens
$5.00
(2)
p. 76
DH15012
Light Meter
$174.00
(1)
$29.00
Total Cost = $1040.18
From the Edmund Optics America Optics & Optical Instruments Catalog, 2005
www.edmundoptics.com
Catalog Location
Number
Name
Price Quantity
p. 133
G53­027
Rotary Assem. w/hold.
$174.10
(2)
p.120
G56­929
Bench Plate
$60.00
(1)
p.128
G03­649
Post Holders 6.0”
$23.10
(2)
p. 128
G36­499
Mounting Posts 4.0”
$10.00
(2)
p. 114
G46­073
Trans. Grating
$123.40
(1)
p. 144
G56­879
4”x4” pos. lift
$146.00
(1)
p.26
G45­298
dbl.convex 40cm fl lens
$33.80
(1)
Total Cost = $571.40
Grand Total = $1610.58