Why do thin films display colourful interference patterns?

Thin films display colourful interference patterns due to the phenomenon of light wave interference.

When light waves encounter a thin film, such as a soap bubble or an oil slick, they split into two. One part of the light wave reflects off the top surface of the film, while the other part enters the film and reflects off the bottom surface. These two reflected waves then recombine, and depending on their phase relationship, they can either interfere constructively or destructively.

The phase relationship between the two waves depends on the thickness of the film and the wavelength of the light. If the film is a whole number of wavelengths thick, the two waves will be in phase and will interfere constructively, resulting in a bright spot. If the film is a half-integer number of wavelengths thick, the two waves will be out of phase and will interfere destructively, resulting in a dark spot.

The colour of the interference pattern is determined by the wavelength of the light. Different wavelengths of light correspond to different colours, with shorter wavelengths appearing blue and longer wavelengths appearing red. When white light, which contains all wavelengths, encounters a thin film, different wavelengths interfere constructively at different thicknesses. This results in a colourful pattern as different parts of the film reflect different colours.

The phenomenon of thin film interference is a fascinating demonstration of the wave nature of light. It shows that light waves, like all waves, can interfere with each other, resulting in patterns of constructive and destructive interference. This is a fundamental concept in physics, and understanding it can help you to understand many other phenomena, from the colours of butterfly wings to the operation of lasers and fibre optic cables.