Physics 1809 Optics 3: Physical Optics
Purpose of this Minilab
• Experiment with and learn about
- Light intensity
- Polarization
- Diffraction
- Interference
Physics 1809 Optics 3: Physical Optics
WARNING – Lasers Used in this Lab
Lasers can cause permanent damage to the eye.
Do not look directly into the laser beam!!!
Do not aim the laser towards others!!!
Physics 1809 Optics 3: Physical Optics
Activity 1: Light Intensity
Light (and other electromagnetic radiation) carries energy.
Energy
Power 
Time
Power
Energy
Intensity 

Area
Area Time
Physics 1809 Optics 3: Physical Optics
Activity 1: Light Intensity
Example: The Sun
The sun radiates 4x1026 Joules of energy every second.
The sun is 1.5x1011m (93.2 million miles) away from the
earth.
What is the intensity of solar radiation
on the solar panel of a satellite?
Physics 1809 Optics 3: Physical Optics
Activity 1: Light Intensity
Energy 4 x1026 J
Power emitted by the sun 

 4 x1026W
Time
s
Earth with satellite (they both are about the same distance
from the sun).
R
All the solar power must pass through a virtual sphere
(with the earth at the surface of that sphere).
The power from the sun is spread out over the
surface area of that sphere (4R2).
Power 4 x10 26W
4 x10 26W
Radiation intensity at satellite 


2
Area
4 R
4 1.5 x1011 m


2

1600W
m2
Note: Due to reflection at the earth’s atmosphere only 250W/m2 arrive at the
earth’s surface.
Physics 1809 Optics 3: Physical Optics
Activity 1: Light Intensity
Photometer: Compares the light intensities entering the two side windows.
Side windows
Look through the eyepiece in the center:
Unequal color in the two half circles indicates different light intensities.
Physics 1809 Optics 3: Physical Optics
Activity 1: Light Intensity
Equal colors in the two half circles indicates equal light intensities.
Physics 1809 Optics 3: Physical Optics
Activity 1: Light Intensity
You can attach a variable filter disk to one side to vary the intensity on that side.
4 different filters are available: 100%, 75%, 50%, and 25% (% indicates the amount
of light transmitted by the filter).
Physics 1809 Optics 3: Physical Optics
Activity 1: Light Intensity
1.1 Measure how the light intensity changes as the light source is placed further
and further away.
Pasco light
source
Point source (hole)
on this side
Photometer
filters
Optics mounts
(empty lens holders)
Flash Light
Optics Bench
Leave some room (maybe 10cm)
between filter and flash light
Physics 1809 Optics 3: Physical Optics
Activity 1: Light Intensity
Optics Bench
r
Procedure for Activity 1.1:
1.
2.
3.
4.
5.
6.
7.
Rotate filter to 100% I0 setting ( = no filtering).
Move point source such that photometer shows even color.
Record distance r.
Rotate filter to 75% I0 setting.
Move point source such that photometer shows even color.
Record distance r.
Etc..
I (arb. units) r
1
0.75
0.50
0.25
…
…
…
…
r2
…
…
…
…
I
I
r
r2
Physics 1809 Optics 3: Physical Optics
Activity 1: Light Intensity
Question 1: What is the relationship between intensity and distance from a point
source?
Hint: Think about the example we gave with the sun.
Physics 1809 Optics 3: Physical Optics
Activity 1: Light Intensity
1.2 Measure how much light intensity is transmitted by a polarizer.
Add a polarizer.
Pasco light
source
Point source (hole)
on this side
Photometer
Filters
(Don’t change distance of
flash light to photometer)
Optics Bench
r
1.
2.
3.
4.
5.
Insert a polarizer between photometer and flash light (but do not change the distance
between photometer and flash light).
Select 100% filter.
Move the Pasco light source until photometer shows equal intensity.
Record distance r.
Use I versus r (or I versus r2) table to determine what I is with polarizer inserted..
Physics 1809 Optics 3: Physical Optics
Activity 2: Polarization
Light has wave characteristics.
y
Electric field vectors
a short time later
x
Direction of propagation
z
Electric field
vectors
Physics 1809 Optics 3: Physical Optics
Activity 2: Polarization
Now looking at the electric field vector at one particular point in space in the
direction of propagation (light travels “into the screen”):
y
y
E
x
even
later
even
later
a little
later
t=0
x
y
E
x
E
Let’s symbolize it as:
x
Etc….
E goes up
and down
x
This light is called “linearly polarized” (in the y-direction).
y
y
E
Physics 1809 Optics 3: Physical Optics
Activity 2: Polarization
x
Linearly polarized
in the y-direction
y
y
y
x
Linearly polarized
in the x-direction
x
Unpolarized light
(a superposition
of many “light waves”
that are polarized in a
random direction).
Physics 1809 Optics 3: Physical Optics
Activity 2: Polarization
A polarizer (often that is a thin sheet of material) only passes light that is
polarized in a certain direction:
Indicates polarizer
orientation.
Polarizer
x
Light before passing
through the polarizer.
y
y
y
x
x
Light after passing
through the polarizer
(no change).
Physics 1809 Optics 3: Physical Optics
Activity 2: Polarization
Polarizer
x
Light before passing
through the polarizer.
y
y
y
x
x
All the light is blocked
by the polarizer.
Physics 1809 Optics 3: Physical Optics
Activity 2: Polarization
Polarizer
x
Light before passing
through the polarizer.
y
y
y
x
x
Only the component of E
that is aligned with the
polarizer passes.
Reduced intensity
Changed direction of
polarization.
Physics 1809 Optics 3: Physical Optics
Activity 2: Polarization
y
Q
x
x
y
y
x
E cos (Q)
E
=
y
y
+
x
x
(vector addition)
E cos (Q)
E sin (Q)
Physics 1809 Optics 3: Physical Optics
Activity 2: Polarization
…and this is why it’s called a “polarizer”
Polarizer
x
Unpolarized light
before passing
through the polarizer.
y
y
y
x
x
After the light passes
through the polarizer:
Light is polarized.
Physics 1809 Optics 3: Physical Optics
Activity 2: Polarization
2.1 Rotate polarizers with respect to each other and observe the intensity
of the light after passing through both polarizers.
Polarizers
eye
Optics Bench
Answer Questions 4 and 5.
Physics 1809 Optics 3: Physical Optics
Activity 2: Polarization
2.2 Measure intensity I versus Q, where Q is the relative angle between the two
polarizer orientations.
Pasco light
source
Point source (hole)
on this side
Photometer
Polarizers
Filters
Optics Bench
Here’s an idea on how to do 2.2 (feel free to improvise otherwise):
1.
2.
3.
4.
Insert two polarizers between photometer and flash light.
Align the two polarizer orientations so they are the same
Put the filter on the side facing the Pasco light source and select the 100% filter.
Move the Pasco light source until photometer shows equal intensity.
Physics 1809 Optics 3: Physical Optics
Activity 2: Polarization
2.2 Measure intensity I versus Q, where is the relative angle between the two
polarizer orientations.
Pasco light
source
Point source (hole)
on this side
Photometer
Polarizer 2 Polarizer 1
Filters
Optics Bench
4.
5.
6.
7.
8.
9.
10.
Select the 75% filter.
Slowly rotate polarizer 2 while observing the photometer.
Find and record all orientations Q of polarizer 2 for which you see equal intensity.
Repeat steps 4-6 for the 50% and 25% filters.
Create a table with two columns: Q and intensity.
Create a graph of intensity versus Q.
Try other plots (e.g. intensity versus cos(Q) or versus cos2(Q)…etc.) to try to find the
relationship between angle and intensity.
Physics 1809 Optics 3: Physical Optics
Activity 3: Diffraction and Interference
Shining coherent light (e.g., laser) through a small slit (or multiple slits) causes
interference (a fancy word for “wave addition”) of the “light waves”.
Wave fronts of light
Dark
screen
Double slit
The wave going through this slit has to travel just a bit
further to get to this particular place on the screen.
The waves from the two slits are out of phase by half
a wavelength.
The two waves annihilate each other.
(“destructive interference”).
 There will be darkness on that place on the screen.
Physics 1809 Optics 3: Physical Optics
Activity 3: Diffraction and Interference
Dark
Bright
The waves going through both slits travel the same distance
to the screen.
The waves from the two slits are in phase.
The two waves add together and have twice the amplitude
(“constructive interference”).
 There will be a bright spot on that place on the screen.
Physics 1809 Optics 3: Physical Optics
Activity 3: Diffraction and Interference
Dark
The light exits the slits in all directions
simultaneously.
A pattern of bright and dark spots appears.
(called “Interference pattern”).
Bright
Dark
Bright
Dark
Bright
Dark
Physics 1809 Optics 3: Physical Optics
Activity 3: Diffraction and Interference
The pattern of interference depends on the slit sizes, slit number, and slit separation, etc..
Single slit
a
(slit width)
Double slit
a
Multiple slits
d: separation between slits
3.1 Look at interference patterns of:
1) Single slits (use different slit widths) (Q7).
2) Double slits (use different slit separations) (Q8).
3) Multiple slits (keep a and d constant and vary number of slits) (Q9).
Physics 1809 Optics 3: Physical Optics
Activity 3: Diffraction and Interference
Laser
Disk with different slit patterns
(rotate to select).
Laser light
Optics Bench
Laser power supply
Screen
Physics 1809 Optics 3: Physical Optics
Activity 3: Diffraction and Interference
Hint for Question 10:
For a double slit there is not only interference between the two slits but also
within each slit (each slit has its own single slit interference).
 Think of superposition of two effects.
Physics 1809 Optics 3: Physical Optics
Activity 3: Diffraction and Interference
3.2 Determine the wavelength of the laser light.
d


mD
y
2nd order maximum (m=2)
Bright
Dark
1st order maximum (m=1)
y (for m=1)
0th order maximum (m=0)
D
Bright
Dark
Bright
Dark
1st order maximum (m=1)
Bright
Dark
2nd order maximum (m=2)
multi slit
Bright
Physics 1809 Optics 3: Physical Optics
Activity 3: Diffraction and Interference
d


mD
y
Bright
Dark
1st order maximum (m=1)
Bright
Dark
Hint:
It is more accurate to
measure 2y and
then divide by 2.
2 y (for m=1)
Bright
Dark
1st order maximum (m=1)
Bright
Dark
Bright
Physics 1809 Optics 3: Physical Optics
Activity 3: Diffraction and Interference
3.3 Determine the distance d between the “grooves” of a CD
d
Physics 1809 Optics 3: Physical Optics
Activity 3: Diffraction and Interference
Method: Reflection on grooves produces also interference pattern.
Dark
Bright
CD (with grooves)
reflected light
Dark
Bright
screen
Dark
Laser
Bright
Dark
screen behind laser
D
Physics 1809 Optics 3: Physical Optics
Activity 3: Diffraction and Interference
Optics mount
Screen
CD attached
Laser light
Laser
Optics Bench
 10 cm
 10 cm
Physics 1809 Optics 3: Physical Optics
Activity 3: Diffraction and Interference
Again: Use d  m D

y
to determine d.
Question 15:
How many grooves are on the CD?
Yes, technically there is only 1 groove on the CD
that snakes its way from the outside to the center.
The proper question you should answer is:
How many times does this groove go around
the CD?
Physics 1809 Optics 3: Physical Optics
Using the Desk Lamp
Lamp Plug (black) must be plugged
into dimmer plug.
Dimmer plug (white) must be plugged
into power outlet.
Dimmer
On/Off
switch
of lamp