StellarThin-Film Interference
When light enters a material that is of similar thickness to its wavelength a special kind of interference occurs called thin-film interference. This happens in a similar way to total internal reflection where some of the light is reflected off the top surface and some it passes through to reflect off the bottom surface.
As seen in the image above, a single ray of light ends up producing two separate rays of light. These light rays can interfere with each other which causes bands of light to appear. This only occurs in thin-films because the light ray travels a very short distance. If the film medium was thick, the second ray would be too far away to cause interference.