Why do soap bubbles show rainbow colors?
Short answer
Light reflects from the extremely thin liquid film of a soap bubble and the reflected waves overlap. Because the film is not the same thickness everywhere, different colors become bright in different places.
How it works
A bubble has an outer surface and an inner surface. When light reflects from both surfaces, the two light waves overlap. Some colors are strengthened by the overlap, while others are weakened. This is called thin-film interference.
The film’s thickness and the angle of the light change across the bubble. That is why colored bands move when the bubble shifts or the film drains.
Key point
This is different from the way raindrops create a rainbow. A soap bubble’s colors come from interference between reflections from a thin film, not from a raindrop separating light by refraction.