1. Diffraction – waves bend as they pass barriers
Eg. Radio waves around hills
light waves through narrow slits
Long wavelength (small f) diffract more than short wavelengths (high f)
Narrow slit
So blue light diffracts less than red light.

2. Interference of Light – Young’s Double Slit Experiment
Click Here
Constructive interference – waves are in phase so add to give double amplitude wave (light, loud, rough). Maxima + =
Destructive interference – waves are 180o out of phase so cancel out to give no wave (dark, quiet, calm). Minima
Click here + =

3. ½ Path difference – waves 180o out of phase destructive
Slit 2
No path difference – waves in phase constructive
½ Path difference – waves 180o out of phase destructive
Slit 1

4. In general
Constructive interference
Path difference = n n = 0,1,2,3…. n is the order of the fringe
Destructive interference
Path difference = (n + ½) n = 0,1,2,3….
Tutorial Questions 5 to 10
In the following experiment monochromatic, coherent light is used
Monochromatic – single frequency (single colour)
Coherent - constant phase difference at the sources (usually they are in phase at the sources)

5. Young’s Double Slit Experiment to find the Wavelength of Light
screen
Laser
Monochromatic
coherent
Double slit 3 2 1
d = distance between slits D = distance from slits to screen
xn = distance between middle and nth fringe
n = order of fringe n = 0,1,2,…
 = d xn nD

6. Diffraction Grating 
Eye or lens needed to focus light d 
d = distance between slits
Path difference
d sin  = path difference
For constructive interference d sin  = n
For destructive interference d sin  = (n + 12) 
n = 0,1,2… n is the fringe order
Tutorial Questions 11 to 14

7. Central maxima
n =1
n = 2
n = 1
screen
Diffraction Grating - interference
Several spectra
Central fringe white
Prism - Refraction
One spectrum only
Higher frequencies/smaller wavelength refracted more
Tutorial Questions 15 to 17
click

8. Word List
Phase monochromatic coherent
Slit fringe order
Maxima minima diffraction
Refraction interference reflection