Coloured light interference pattern on a thin film of oil floating on water. Interference effects commonly arise by reflection in thin films; when the oil film thickness is comparable to the wavelength of light, light waves reflected from the back of the film may be out of phase with light striking the front of the film, and interference results. Colour arises when the incident light is white, and when variations in thickness of film produce interference in some of the component wavelengths of white light. When such wavelengths are extinguished, the residual light is coloured.