page 1
Light Amplification by Stimulated Emission of Radiation
Lasers are all around us in many places you
might not realize. Besides being useful for pure
science like in a physics lab, lasers are found in
many real-world applications.
Just a few examples where you
find lasers:
The grocery store
The Doctor’s office
Manufacturing
Telecommunication
The Moon
Weapons
page 2
Lasers in the Supermarket
If you’ve ever had a barcode scanned at Wegmans or anywhere else, you’ve
experienced a laser firsthand.
The scanner measures the brightness of the reflected light and converts this
information into numbers and letters.
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Lasers in the Doctor’s Office
Laser-Assisted in
SItu Keratomileusis
Lasers are also used by dentists to fill
cavities, and by doctors as a scalpel.
Lasers have the advantage of being more
precise and much less invasive than a
standard scalpel. Lasers have become
widespread in the treatment of cancer
(tumor removal)
page 4
Lasers in Manufacturing
Lasers can cut and weld complex structures out of both hard and soft materials, and at
much smaller scales than traditional welding. This type of welding/cutting can be
computer controlled for ultra high precision.
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Lasers in Telecommunication
Fiber-optic communication is one of the most important contributions from physics
to our daily lives.
Telephone, internet, television are all transmitted using fiber-optics.
Laser light is used as a carrier for different types of information, which can then be
sent huge distances with very little signal degradation and at high speeds. Anyone
with Verizon FiOS is using this technology. In fact, fiber-optic cables are
underground all around us.
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Lasers on the Moon
Apollo astronauts left reflectors on the
surface of the moon. By sending
millisecond laser pulses at the reflector
and measuring the time it takes to reflect
the signal back, scientists have been able
to measure the distance to the moon.
This is how we know the moon is
moving away from us by 1.5 inches per
year.
This also takes place on Earth. The
surveyors you see on the side of the
road are using laser rangefinding
equipment to create detailed maps,
some times even in 3D.
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Lasers in Weapons
Lasers can even be used to blow stuff up. In fact, phasers and ray guns may not be
far off. On February 11, 2010 in a test at Point Mugu Naval Air Warfare CenterWeapons Division Sea Range off the central California coast, a Boeing 747
successfully destroyed a ballistic missile in flight with a laser.
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Lasers in Weapons
UB has several labs, including some in this building, that contain powerful lasers such
as this Carbon Dioxide laser. The surface of a test target is instantly vaporized and
bursts into flame carbon dioxide laser emitting tens of kilowatts of far infrared light.
Note the operator is standing behind sheets of plexiglas, which is opaque in the far
infrared.
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How these Lasers Work
The lasers you’ll use today are Helium-Neon (He-Ne) lasers. They contain a reservoir
filled with low pressure Helium and Neon gas. The other important part is a highly
reflective mirror at one end.
When you turn the power on, an electric discharge inside the laser excites the lightweight helium atoms, just like you saw in the Hydrogen Balmer lines experiment.
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How these Lasers Work
Because the Helium atoms are light, they begin to move very fast inside the reservoir when they
are in their excited state. If an excited Helium atom collides with a Neon atom, its energy gets
transferred to the Neon atom. It just so happens that one of the excited states of Neon has
almost the exact same energy as the excited state of Helium.
The excited electron in the Neon atom now relaxes to an intermediate energy state of its own,
and the excess energy is lost to light of the red color that you see. This light reflects off the
mirror inside and if the photon then collides with yet another Neon atom it bumps the electron
back into the excited state so it can relax again and emit even more light. This is stimulated
emission and it’s what actually makes a laser a laser.
page 11
Diffraction of Light: Particle or Wave?
Imagine light as a stream of particles like tennis balls. If you throw the tennis
balls at a set of holes in the wall and watch where they hit on the other side,
you’d see them only hit at the places where the balls made it through the gaps
in the wall, and nowhere else.
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Diffraction of Light: Particle or Wave?
What you’ll see today is that this is not the case when you shine a laser
through such a pattern. These results can only be explained by describing
light as a wave and using the principles of wave interference we talked
about on Monday.
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Diffraction of Light: Particle or Wave?
In fact, the wave nature of light leads to all kinds of bizarre interference patters, many of
which you’ll be looking at today:
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Diffraction of Light
In this lab you will complete the following tasks:
a. Study the diffraction pattern by a double slit
b. Study dispersion of light using a diffraction grating.
You will use a grating to measure the wavelength of
the red line emitted by a Helium-Neon laser
c. Study the diffraction pattern by a two-dimensional
array of holes
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Section V-1: Diffraction from a double slit
d
Double
slit
θ
laser
L
Optical bench (top view)
screen
Use the setup shown in the figure above.
Use the double slit with slit distance d = 0.25 mm.
Use L = 200 cm. Place a paper sheet on the screen and trace the
diffraction pattern shown in the figure below
Double slit diffraction pattern
-4 -3 -2 -1 0 1 2 3
4
page 16
Diffraction from a double slit
Double
slit
x
θ
laser
L
Optical bench (top view)
Positions of diffraction maxima
m L
xm 
d
m  0,  1,  2, ...
Double slit diffraction pattern
-4 -3 -2 -1 0 1 2 3
x-3
screen
x3
4
x3  x3
x3 
2
3 L
d exp 
2 x3
d
V2. Diffraction Grating
d
Incident beam
Grating
d sin   m
Transmitted
beam
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A diffraction grating consists of identical parallel
openings in the form of long slits on an opaque
screen. These slits allow the transmission of light.
The distance between adjacent is slits is equal to
d . Consider a light beam of wavelength 
incident on the grating at right angles. The
transmitted (also called diffracted ) light propagates
past the grating only at certain directions defined
by the angle  of the diffracted beamd with the
normal to the grating. The allowed angles are
given by the equation: d sin   m
Here m is an integer and can take the values:
m  0,  1,  2, .... Different wavelengths
propagate along different angles. Thus the grating
separates (disperses) the various wavelengths in
a light beam.
x2
ruler
m=0
m = -2 m = -1
B'
A'
x1
m=1
m=2
A
B
C
θ1
L
θ2
V -1
d
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Measure the wavelength He  Ne
of the red line from a Helium-Neon laser
using a diffraction grating.
G
Grating
The diffracted laser spots on the ruler
is shown in the lower figure.
We measure the distance x1 between A and C
He-Ne
laser
m = -2
B'
m = -1 m = 0
.
A'
C
m=1
A
We measure the distance x2 between B and C
m=2
B
x2
x1
ruler
m=0
m = -2 m = -1
B'
A'
m=1
m=2
A
B
C
θ1
L
θ2
d sin   m
G
d  1.67 103 nm
L  50 cm
Grating
x 
1  tan 1  1 
L
d sin 1  He Ne
He-Ne
laser
m = -2
B'
page 19
d
m = -1 m = 0
.
A'
C
m=1
A
m=2
B
x 
 2  tan 1  2 
L
d sin  2  2He Ne
page 20
d
V-2: Diffraction from a
square array of holes
d
Screen
Diffraction pattern
Square
array
O
P
L
θ
Laser
Use L = 400 cm
Use
the square array with
d = 0.1 mm
d sin  


mx 2  m y 2 
mx , m y  0, 1, 2, ...
Place a sheet of paper on
the screen on which you
will trace the diffraction
pattern
Diffraction pattern
y
(0 , 2)
(-1 , 2)
(-2 , 1)
(0 , 1 )
xD
(-1 , 0)
O
(-2 , -1)
(-1 , -1)
(0 ,- 1)
(-1 , 2)
(1 , 0)
xC
(1 , 1)
d
(0 , -2)
(2 , 1)
A
xA
(-2 , 0)
C
Diffraction from a
square array of holes
d
(-1 , 1)
D
page 21
(1 , 0)
(1 , -1)
xB
(2 , 0)
x
(2 , -1)
B
(1 , -2)
x A  xB  xC  xD
x
4
d exp
 2

x/L