Why do soap bubbles show interference colours?

Soap bubbles show interference colours due to the phenomenon of thin film interference of light waves.

When light waves encounter a soap bubble, they are reflected off both the outer and inner surfaces of the thin soap film. This results in two waves: one reflected from the outer surface and one from the inner surface. These two waves then interfere with each other. Depending on the thickness of the soap film and the angle of incidence of the light, the two waves can either constructively or destructively interfere, leading to the colourful patterns observed.

The colours seen are a result of the varying thickness of the soap film. The thickness of the soap film changes due to gravity and evaporation, which causes the colours to shift and move. The colours are also influenced by the angle at which you view the bubble. As you move around the bubble, the angle of incidence of the light changes, which alters the path difference between the two waves and therefore changes the interference pattern.

The phenomenon of thin film interference is not unique to soap bubbles. It can also be observed in oil slicks on water and in the coloured patterns seen in peacock feathers. However, soap bubbles provide a particularly beautiful and easily observable example of this phenomenon.

In terms of the physics involved, the interference of light waves is a result of the wave nature of light. When two waves meet, they can either add together to create a wave of greater amplitude (constructive interference) or cancel each other out to create a wave of lesser amplitude or even no wave at all (destructive interference). The colour seen in each part of the bubble is determined by which wavelengths of light are constructively interfered and therefore amplified.

In conclusion, the interference colours seen in soap bubbles are a beautiful demonstration of the wave nature of light and the phenomenon of thin film interference. They provide a visually stunning example of some of the fundamental principles of wave optics.