1. 15 – 2: Diffraction
Another property that light exhibits is diffraction.
Diffraction: the bending of a wave around an obstacle.
Diffraction occurs when light passes through very small apertures or near sharp edges.

2. Light We can get an interference pattern when there are two slits.
We will also get an interference pattern with a single slit.
Light

3. wsinq = ml (m = ±1, ±2, ±3 …) Destructive Interference
Diffraction can be explained using Hugyen’s Principle which states that each point along the slit (of width W) can be considered to be a point source of light.
There will be a path difference between light leaving the top of the slit and the light leaving the middle. This path difference will yield an interference pattern. P
Light q
Dl = (w/2) sinq
wsinq = ml (m = ±1, ±2, ±3 …) Destructive Interference

4. Single Slit Diffraction
Notice that the central maximum is twice as wide as the secondary maxima.
Position on screen:
y = L tanq
m=1
m=-1 L

5. Slit width = d Slit width = 0.8d
Changing the width of the opening stretches or compresses the diffraction pattern.
Slit width = d
Slit width = 0.8d

6. Diffraction Grating Each slit is a source of a wave.
Observe the outgoing wave at an angle q.
The contributions from all slits add up coherently if
dsinq = ml, (m = 0, 1, 2,…)

7. Sharpening of Diffraction Pattern
The diffraction grating reduces the width of each maximum by a factor 1/N, where N is the number of slits.
Diffraction pattern with N = 5

8. Light as a Wave Also Explains Doppler Shift

9. We are Familiar With the Shift in Sound Waves

10. Doppler Shift in Sound Waves
(a) Sound of a train moving towards us
is higher pitched
(b) Sound of a train moving away from us
is lower pitched

11. Doppler Shift in Light Waves
If source approaches, light appears bluer than it is.
If source recedes, light appears redder than it is.

12. Doppler Shift Wavelength is shorter when approaching Stationary waves
Wavelength is longer when receding

14. Doppler Shift / Redshift
Redshift, z, is a non-relativistic
approximation to the Doppler shift Dl l =
l - lo lo v c z