1. WAVE OPTICS & LASER

2. WAVE OPTICS & LASER Interference Air-Wedge – Theory and Applications
Types of LASER
Nd:YAG LASER, CO2 LASER, Semiconductor LASER (homojuntion)
Application : Holography
Fiber Optics – Principle
Types of Optical Fibers
Fiber Optics Communication System

3. WAVE OPTICS Wave Optics deals with different optical phenomena
Interference
Diffraction
Polarisation

4. INTERFERENCE
A phenomenon in which two waves superpose to form a resultant wave of greater amplitude or lower amplitude
Interference effects can be observed with all types of waves
Light waves
Radio waves
Acoustic waves
Surface water waves
Matter waves

5. Path Difference and Phase Difference
Path difference and phase difference will be meaningful for waves of same frequency
They are used to determine constructive and destructive interference in waves
Path difference (measured in terms of wavelength)
Actual measurable difference in distance that two waves travel from a source to a common point
Path difference between the red and blue wave = λ/ 4
Phase difference (measured in radians)
Difference in the phases of the two sinusoidal waves of same frequency
The blue wave leads the red wave by a phase difference of π/2 (90 degrees)

6. Path Difference and Phase Difference
At time t = 0
Blue wave displacement = 0
Red wave displacement = – A
At π/2
Blue wave has maximum displacement = + A
Red wave displacement = 0
Blue wave is leading by a phase difference of π/2 and path difference of λ/4
One oscillation is completed in 2π radians which is equivalent to wavelength λ
(Path difference of one wavelength (λ) is equal to phase difference of 2π radians)

7. PD = 5λ – 4.5λ
= 0.5λ = ½ λ
PD = 5.5λ – 4.5λ
= λ
Waves follow different paths from the slits to a common point on a screen
(a) Destructive interference
One path is a half wavelength longer than the other
The waves start in phase but arrive out of phase
(b) Constructive interference
One path is a whole wavelength longer than the other
The waves start out and arrive in phase

8. Constructive Interference

9. OPTICAL INTERFERENCE
Formation of bright and dark bands resulting from the super-position of two coherent light waves
Constructive Interference (Bright band)
Path difference = n λ
Phase difference = n 2π
Destructive Interference (Dark band)
Path difference = (2n – 1) λ/2
Phase difference = (2n – 1)π
Coherent light waves
Light waves have
same wavelength
same amplitude
constant phase difference

10. Optical Interference Band Width or Fringe Width
Distance between two successive bright bands or dark bands

11. Optical Interference OPL = μ d
To obtain clear and broad interference bands
Wavelength of light should be larger
Light sources should be closer and narrower
Distance between the screen and the coherence sources should be larger
OPTICAL PATH LENGTH
Product of the geometric length (d) of the path light follows through a system and the refractive index (μ) of the medium through which it propagates
OPL = μ d

12. Polarisation
Optical phenomenon in which the light vibrations are restricted to take place in a single plane
An ordinary light is unpolarised light since it can vibrate in all directions in a plane perpendicular to the direction of propagation of light

13. Interference at a Wedge-Shaped Film
ABC - A wedge shaped film of refractive index (μ) with very small angle
A parallel beam of monochromatic light is incident on the upper surface
The surface is viewed by reflected light through a travelling microscope
Alternate dark and bright bands can be observed

14. Interference at a Wedge-Shaped Film
An air wedge is formed by two plates of glass separated at one end
No phase change
180o phase change

17. Interference effect is between light reflected from the bottom surface of the top plate and light reflected from the top surface of the lower plate
Bottom of the upper plate → glass-to-air boundary (μglass > μair)
No phase change upon reflection
Top of the lower surface → air-to-glass boundary (μair < μglass)
There is a 180° phase change upon reflection from this surface

18. Interference at a Wedge-Shaped Film
Point P will appear dark and a dark band will be observed across the wedge, if
Point P will appear bright and bright band will be observed across the wedge, if

19. Interference at a Wedge-Shaped Film
If the nth dark fringe is formed at P
Similarly, for the (n+1)th dark band which is formed at Q at a distance x2 from A

20. Interference at a Wedge-Shaped Film
Fringe width (β)
So, if we consider any two consecutive bright fringes, β will be the same

21. Interference at a Wedge-Shaped Film
A wedge shaped air film can be obtained by inserting a thin piece of paper or hair or thin wire between two glass plates
For air film
t – thickness of paper or hair or thin wire
x – distance from the edge to the paper or hair or thin wire

22. Thickness of a paper or wire or hair
A wedge shaped film is obtained by inserting a thin paper or thin wire or hair between two parallel glass plates (optical flats)
Sodium vapour lamp is used as the light source
Light is incident on the glass plate inclined at 45⁰ to the horizontal
Glass plate makes the light to fall normally on the optical flats by reflection
By adjusting the distance between the microscope objective and optical flats, we can get the interference fringes in the microscope eye piece

23. Thickness of a paper or wire or hair
By coinciding one of the bright fringes with the vertical crosswire, the reading is noted in the horizontal scale of microscope
After crossing of five bright fringes, once again the reading is noted
The value of fringe width is calculated
Distance “x” from the edge where the two plates touch each other to the paper or thin wire is measured
Then the thickness t can be calculated from the formula

25. Testing of flatness of a surface

26. Testing of flatness of a surface
Not smooth surface
Incident light
Smooth surface
Incident light

27. Bright Band
Path Diff = nλ
Phase Diff = n2π
Dark Band
Path Diff = (2n-1)λ/2
Phase Diff = (2n-1)π
Interference
Wave Optics
Thickness of thin
sheet or wire or hair
Testing of Flatness of a surface
Air Wedge shaped film