Interference summary

Division of wave front:

Interference maxima for equally spaced slits with equal width (doube slit, multiple slits, grating):

d sinθ = mλ    (d = slit spacing)

Division of amplitude:

Thin film interference is the interference of light waves reflecting off the top surface of a film with the waves reflecting from the bottom surface.

Reflection of light from a surface introduces a phase shift of 0 or π.

For constructive interference the waves reflected from the top and bottom surfaces of the film have to be in phase and for destructive interference they have to be 180^o out of phase.

Example:  Light normally incident on an oil film of thickness t on water

Constructive interference:  2 n_oil t = (m+½)λ, m = 0,1,2,…

Destructive interference:   2 n_oil t = mλ, m = 1,2,…