Why do we observe coloured fringes in thin films?

We observe coloured fringes in thin films due to the phenomenon of interference of light waves.

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. Depending on the thickness of the film and the angle of incidence of the light, the two waves may either reinforce each other (constructive interference) or cancel each other out (destructive interference). This interference of light waves results in the appearance of coloured fringes.

The colour of the fringes depends on the wavelength of the light and the thickness of the film. When the film is very thin, the path difference between the two waves is small, and they interfere constructively, producing bright fringes. As the thickness of the film increases, the path difference also increases, and the waves begin to interfere destructively, producing dark fringes. The sequence of bright and dark fringes forms a pattern known as an interference pattern.

The phenomenon of interference is a fundamental aspect of wave behaviour and is not limited to light waves. It can be observed with all types of waves, including sound waves and water waves. However, it is most easily observed with light waves due to their short wavelength and the ease with which they can be manipulated.

In summary, the coloured fringes observed in thin films are a visual manifestation of the interference of light waves. This phenomenon provides a fascinating insight into the wave nature of light and has numerous practical applications, from the creation of anti-reflective coatings to the measurement of very small distances and thicknesses.