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.

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 + =

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

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)

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

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

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

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