Thin-Film Interference
Definition
The rainbow colors in soap bubbles and oil slicks — light waves reflecting off the film's two surfaces interfere.
Technical definition
Interference between reflections from the front and back surfaces of a thin film. The constructive condition depends on film thickness, wavelength, refractive index, and a half-wave phase flip at the higher-index boundary: 2nt = (m + ½)λ for bright reflection (air film).
Example
A soap bubble's swirling colors: different thicknesses across the bubble reinforce different wavelengths — the film acts as a thickness map painted in color.
Common confusion
The colors come from interference, not pigment — there is no dye in a soap bubble. And the pattern changes as the film drains and thins, which is why the colors swirl and shift.