This document discusses the conditions for constructive and destructive interference that occur when light reflects off thin film interfaces. Constructive interference occurs when the optical path difference between the reflected waves is equal to integer multiples of the wavelength. Destructive interference occurs when the path difference is equal to half-integer multiples of the wavelength. The phase shift of the reflected waves impacts whether the conditions are for constructive or destructive interference.

26. The amplitude of the wave reflected from
the interface is proportional to the
amplitude of the incident wave and is given
by

28. If the film has thickness t the light is at normal incidence and has
wavelength in the film; if neither or both of the reflected waves from
the two surfaces have a half-cycle reflection phase shift, the conditions
for constructive and destructive interference are
If one of the two waves has a half-cycle reflection phase shift, the
conditions for constructive and destructive interference are reversed: