Thin Films

Light which partially passes through and partially reflects off a thin film of material can also interfere with itself
If a light ray in one substance reflects off an interface to a material with a higher index of refraction, its phase is reversed
If a ray which reflects off the front surface a film, and a ray which reflects off the back surface of a film, recombine in phase, one sees the reflected light; if they recombine out of phase, no reflection appears
The reflection from a thin film can depend strongly on wavelength