Coated lens surfaces and greasy water surfaces show us the colors of rainbows. This is because of light interference.

Lens coating, using a thin, transparent film, is designed to decrease reflections while increasing transparent light. This film is designed by calculating the appropriate thickness and refractive index to ensure that light of a specific wavelength (the design wavelength) reflected from the surface of the film and light reflected from the border between the film and the glass interfere and neutralize one another. However, it is not always possible to guarantee zero-percent reflection of light with wavelengths different from the design wavelength. The eye therefore perceives these light waves as purplish reds and blues.

Although the physical conditions are different, the colors you can see on a thin film of oil floating on a surface of water are also the result of mutually interfering light waves.