1. Chapter 5: Wave Optics
How to explain the effects due to interference, diffraction, and polarization of light?
How do lasers work?

2. Wave Optics
Effects due to interference, diffraction, and polarization can not be explained by geometric optics.
Wave nature of light was demonstrated by Young’s double slit experiment (1820).
In phase waves
lead to constructive
interference
Out of phase waves
lead to destructive
interference

3. 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, …, (n+1/2)
Path difference P = (r2 – r1)

4. Example: Two Wave Interference
Path difference = 1 wavelength
Path difference = 1/2 wavelength

5. Two Wave Interference (Contd.)
Condition for constructive interference:
n = 0, 1, 2, …

6. Two Wave Interference Pattern
Intensity y
n=0
n=1
n=2

7. 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.

8. 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.

9. 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.
primer/java/imageformation/
gratingdiffraction/index.html
2 slits
8 slits
16 slits

10. 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

11. Review Problem
A grating has 5000 lines/cm. A second order maximum is observed at What is the wavelength of light?
500 nm

12. LASER: Light Amplification by Stimulated Emission of Light
Stimulated emission process was predicted by Einstein in 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.

13. 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 current…etc.
PUMP
Normal condition:
Thermal equilibrium
Population inversion
achieved by “pumping”

14. Laser Operation in a 3 Level System
Excited
1. Pumping: Excites atoms to highest level.
Reservoir
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

15. Light is amplified in a “resonant cavity” between two mirrors.
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
100% Reflecting
Mirror
90% Reflecting
Mirror

16. 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.

17. Types of Lasers
Solid state lasers, gas lasers, dye Lasers, semiconductor (diode) lasers.
Laser Type
Wavelength (nm)
Free Electron
UV, X-ray??
Excimer: Argon fluoride
193
Nitrogen
337
Argon Ion (blue)
488
Argon Ion (green)
514
Helium Neon
633
Rhodamine 6G Dye (tunable)
Ruby (CrAlO3)
694
Nd:YAG
1064
Carbon Dioxide
10600

18. Holography (3D Imaging) Conventional Photography
3D Object
Film
Conventional Photography
Holography
Relative intensities are recorded on film
Interference pattern is recorded on film
“Phase” information about waves is lost
“Phase” information is retained in the interference pattern

19. 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.

20. Light is a “transverse” electromagnetic wave.
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.

21. Production of Polarized Light
Selective Absorption:
Note: Optically “active” materials can change the polarization direction. Example: Sugar solution, DNA, liquid crystals…etc.

22. Production of Polarized Light (Contd.)
Reflection:

23. Production of Polarized Light
Scattering:
Light scattered in a perpendicular direction
is partially polarized!

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