Soapy water is usually almost clear. The coloured bands on a bubble are not pigments but the result of thin-film interference. When white light reaches the film, some reflects from the outer surface while some enters the film and reflects from the inner surface. The two portions travel different distances, and reflection can also shift their phase. When they meet again, some wavelengths reinforce one another while others cancel, so particular colours reach our eyes more strongly.
Film thickness is one of the key factors. A soap film is on the scale of visible-light wavelengths and is not uniformly thick. Liquid drains downwards under gravity, while evaporation continually thins the film. Different parts of one bubble, and the same part from one second to the next, can therefore reinforce different wavelengths. The moving bands do not mean that the light has been dyed inside the film.
A dark patch often appears near the top shortly before the bubble bursts. When the film becomes sufficiently thin, reflections from its two surfaces largely cancel across visible wavelengths, so less reflected light reaches the eye. The film has not yet vanished; the darkness is produced by how the reflected waves combine. Oil films can show similar colours, but the exact pattern also depends on refractive index, thickness, viewing angle and illumination, so colour alone is not a precise thickness gauge.
https://openstax.org/books/university-physics-volume-3/pages/3-4-interference-in-thin-films
Discover more from Geoffrey Chen
Subscribe to get the latest posts sent to your email.