How do overlapping reflections make colors in a thin film?

A thin oil film on water can shimmer with colors even when the oil has no bright pigment. These are interference colors. Some incoming light reflects from the upper surface, while some enters the film, reflects from the lower surface, and returns outward. The two contributions overlap rather than forming separate colored beams like a rainbow.

Light behaves as a wave. Its phase describes where it is in a repeating cycle. Waves that line up can reinforce one another; waves that oppose one another can weaken or cancel their combined effect. The extra travel through the film changes the phase of the lower reflection.

Reflection itself can also change phase. Refractive index is light’s vacuum speed divided by its speed in the material. The refractive indices are 1.00 for air, 1.50 for oil, and 1.33 for water. The upper reflection flips phase while the lower one does not. Together with the travel distance, this determines which wavelengths are reinforced. For the illustrated 100-nanometer oil film and selected 600-nanometer light at normal incidence, the two ideal reflected waves line up and reinforce.

White light contains many wavelengths, so a real film can strengthen some colors more than others. Changes in thickness or viewing angle alter the pattern. The equal-amplitude wave sketch isolates this mechanism; it does not calculate a realistic perceived color or include every repeated reflection within the film.