The variety of colours in thin films arises as a result of the interference of light waves reflected from the upper and lower boundaries of the films. The wave reflected from the lower boundary has to travel an additional path in comparison with the wave reflected from the upper boundary. Therefore the wave reflected from the lower boundary lags behind the wave reflected from the upper boundary. The extent of this phase lag depends on the thickness of the film at the place where the reflection takes place and on the wavelength of the light in the film material.
Waves corresponding to a given colour, when reflected, acquire a difference in path equal to an odd number of half-wavelengths only at given thicknesses of film. Because of the interference between the reflected rays there will be destructive interference of some colours of the spectrum and reinforcement of others, according to the thickness of the film in a given region. Therefore regions of different thickness of film will appear to be of different colour.
