Chapter 5: Wave Optics

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Chapter 5 Wave Optics How to explain the
effects due to interference, diffraction, and
polarization of light? How do lasers work?
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• Wave Optics
• Effects due to interference, diffraction, and
  polarization can not be explained by geometric
  optics.
• Wave nature of light was demonstrated by Youngs
  double slit experiment (1820).

In phase waves lead to constructive interference
Out of phase waves lead to destructive interferen
ce
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• Two Wave Interference
• What causes two identical waves to become
  in-phase or out-of-phase?
• Path difference between the two waves!
• Waves are in-phase when
• P 0, ?, 2?, 3?, , n?
• Waves are out-of-phase when
• P ?/2, 3?/2, 5?/2, , (n1/2)?

Path difference P (r2 r1)
http//www.physics.northwestern.edu/ugrad/vpl/wave
s/superposition2.html
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Example Two Wave Interference
Path difference 1 wavelength
Path difference 1/2 wavelength
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Two Wave Interference (Contd.)
Condition for constructive interference
n 0, 1, 2,
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Two Wave Interference Pattern
Intensity
y
n0
n2
n1
www.Colorado.EDU/physics/2000/schroedinger/two-sli
t2.html
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Thin Film Interference (Soap Bubbles)
The phase difference of rays reflected from the
top and bottom surfaces depends on the thickness
and refractive index of the film, the angle at
which the light strikes the film surface and the
wavelength of the light.
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Thin Film Interference (Antireflection coating)
The substrate (glass, quartz, etc.) is coated
with a thin layer of material so that reflections
from the outer surface of the film and the outer
surface of the substrate cancel each other by
destructive interference.
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Multiple Wave Interference Diffraction Grating

• Constructive interference occurs only when all
  waves are in-phase.
• Path difference between any two successive waves
  must be nl.
• Condition for interference maxima is,
• Interference pattern has
• sharp peaks.

http//www.microscopy.fsu.edu/ primer/java/imagefo
rmation/ gratingdiffraction/index.html
2 slits
16 slits
8 slits
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Diffraction Grating (Contd.)

• Gratings have hundreds of slits per cm.
• Applications in spectroscopy, crystallography
  etc.

Diffraction pattern from a crystalline solid
Diffraction of light from a CD
Iridescence A diffraction phenomenon
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Review Problem
A grating has 5000 lines/cm. A second order
maximum is observed at 300. What is the
wavelength of light?
500 nm
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• LASER Light Amplification by Stimulated Emission
  of Light
• Stimulated emission process was predicted by
  Einstein in 1916. First laser developed in 1959.
• Photons and atoms can interact via the
  following processes.

Absorption Atom can absorb a photon and become
excited. Spontaneous emission Atom in excited
state will spontaneously emit a photon and
occupy a lower energy state. Stimulated
emission Atom in excited state is stimulated by
a photon to emit another photon and occupy a
lower energy state. Emitted photon has the same
wavelength, phase, and direction as the
stimulating photon.
http//www.colorado.edu/physics/2000/lasers/lasers
2.html
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• Stimulated Emission
• Stimulated emission is more likely under
  population inversion.
• Pumping Process by which energy is supplied to
  excite more atoms to achieve population
  inversion. Atoms can be pumped by photon
  absorption, collisions, electric currentetc.

PUMP
Normal condition Thermal equilibrium
Population inversion achieved by pumping
http//www.colorado.edu/physics/2000/lasers/lasers
3.html
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• Laser Operation in a 3 Level System

Excited
Reservoir
1. Pumping Excites atoms to highest level.
Ground
Excited
2. Fast radiative decay to reservoir creates
population inversion between reservoir and
ground states.
Reservoir
Ground
Excited
3. Seed photon stimulates emission and light
is amplified!
Reservoir
Laser light
Ground
http//www.phys.hawaii.edu/7Eteb/optics/java/lase
r/index.html
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• Light Amplification
• Light is amplified in a resonant cavity between
  two mirrors.
• Photons from stimulated emission bounce between
  mirrors knocking out more photons. Light is
  amplified!

Active Medium
Laser Light
90 Reflecting Mirror
100 Reflecting Mirror
http//www.colorado.edu/physics/2000/lasers/lasers
4.html
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• Properties of Laser Light
• High Power Density At the focus, lasers can be
  thousands of times more intense than the sun!
• Sunlight 1300 W/m2
• Laser 106 W/m2
• High Spectral purity Light is emitted in a
  narrow band of wavelengths. This is due to the
  atomic processes in the active medium.
• Small beam divergence All photons travel in the
  same direction. Typical beam divergence 2 x
  10-5 degrees/m.
• Coherence All the emitted photons
• bear a constant phase relationship
• with each other in both time and space.

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• Types of Lasers
• Solid state lasers, gas lasers, dye Lasers,
  semiconductor (diode) lasers.

http//www.microscopy.fsu.edu/primer/lightandcolor
/java.html
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Holography (3D Imaging)
3D Object
Film
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Holography (2 Step Process)
Reconstruction
Recording
Interference pattern is recorded on film. Need
high resolution (slow) film, long exposure and
vibration free set up.
Interference pattern acts as a diffraction
grating so different orders of maxima and
minima reconstruct the image.
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• Polarization
• Light is a transverse electromagnetic wave.
• Polarization is the orientation of the electric
  field.

• Note
• Natural light is randomly polarized.
• Eye cannot distinguish different
• polarizations.

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• Production of Polarized Light
• Selective Absorption
• Note Optically active materials can change the
  polarization direction. Example Sugar solution,
  DNA, liquid crystalsetc.

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• Production of Polarized Light (Contd.)
• Reflection

http//www.colorado.edu/physics/2000/applets/polar
ized.html
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• Production of Polarized Light
• Scattering
• Light scattered in a perpendicular direction
• is partially polarized!

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• Polarized Light Some Applications
• Mineral characterization.
• Stress / strain fields (visual inspection of
  windshields).
• Polarization microscopy.
• Sunglasses / camera filters.
• LCD displays.
• Polarized art?

http//www.colorado.edu/physics/2000/index.pl
http//micro.magnet.fsu.edu/primer/virtual/polariz
ing/index.html