Interference in Thin Films

Thin films (such as oil films on the surface of water) can exhibit interference effects. Such films have many useful applications, such as anti-reflection coating on camera lenses.

The following concepts are central to understanding interference in thin films:

Phase changes on reflection: a wave reflecting from a region of higher index of refraction is phase-shifted by 180°. There is no phase change for a wave reflecting from a region with lower index of refraction.

Optical thickness: since the wavelength of light decreases when it enters a region with greater index of refraction, the optical thickness nt is greater than the physical thickness t of a film.