Soap bubbles are thin films of soap and water. When light, like sunlight, hits a bubble, it interacts with this film. Light is made of different colours, and each colour behaves differently when it meets the bubble.
The bubble’s film isn’t the same thickness everywhere. As light hits different parts of the bubble, some of it bounces back immediately (reflection), while some goes into the bubble, slows down, and then bounces back (refraction). Depending on the film’s thickness and where you’re looking, different colours bounce back.
Because of this variation in thickness and the way each colour of light interacts with the bubble, we see a mix of colours — a rainbow effect.
The light we see ranges from 400 nanometers (violet) to 700 nanometers (red) in wavelength. The soap bubble’s thickness can be a few hundred nanometers, which affects which colours of light are reflected and refracted, creating the rainbow look.
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