1. Interference and Diffraction
Physics 014
Interference and Diffraction

2. Topics Light as a wave Diffraction Young’s interference experiment
Intensity & Double slit interference

3. Light as a wave Huygen’s Principle:
All points on a wavefront serve as point sources of sperical secondary wavelets.
After a time t, the new position of the wavefront will be that of a surface tangent to these secondary wavelets.

4. Light as a wave

5. Light as a wave

6. Light as a wave

7. Light as a wave The time taken to go from point e to point c
EQUALS the time taken to go from point h to point g, so

8. Light as a wave Remember that for the index of refraction
c=speed of light in vacuum,
v=speed of light in material

9. Light as a wave
Using these expressions we can rederive Snell’s law of refraction.

10. ???
The figure shows a monochromatic ray of light traveling across parallel interfaces, from an original a, through layers of materials b and c, and then back into a. Rank the materials according to the speed of light in them.

11. Light as a wave
Notice that although the wavelength changes between two media,
The frequency does NOT change between two media.

12. Light as a wave
The wavelength change in media means that a phase difference between two light waves can change if they travel through two different media.

13. Diffraction
If a wave hits a barrier with an opening, the part of the wave that passes through the barrier will flare out. (diffract)

14. Diffraction

15. Diffraction

16. Diffraction

17. Young’s Interference Exp.
Young’s double slit experiment displayed a characteristic interference pattern

19. Young’s Interference Exp.
Maxima: bright bands, bright fringes
Minima: dark bands, dark fringes

20. Young’s Interference Exp.

21. Young’s Interference Exp.
Path difference
Maxima
Minima
m=0,1,2,…

22. Young’s Interference Exp.

23. ???
For the diagram, what are ΔL (as a multiple of wavelength) and the phase difference (in wavelengths) for the two rays if point P is (a) a third side maximum and (b) a third minimum

24. Single slit diffraction
Let’s consider plane waves diffracted by a single long, narrow slit of width a on a screen

25. Single Slit Diffraction

26. Single Slit Diffraction

27. Single Slit Diffraction
The minima may be found using,
a sinθ = m λ
for m = 1,2,3, …

29. Single slit diffraction
The intensity for the single slit diffraction pattern is

30. Diffraction by circular apperature

31. Double Slit Diffraction

32. Diffraction Grating

33. Interferometers

34. Interferometers