Question

Colours appear on a thin soap film and soap bubbles due to the phenomenon:
(A) Interference
(B) Dispersion
(C) Refraction
(D) Diffraction

Answer 1

Hint: Have you noticed colours on the surface of the water in a puddle. Have you wondered why it happens? The colours appearing on a thin soap film follow the same phenomenon as the colour in a water puddle. This phenomenon is known as thin-film interference. Let us try to understand the physics behind these seemingly natural yet fascinating processes.

Complete solution:
Streaks of red, blue, orange and green colour are observed because of thin-film interference. It also happens in bubbles. If you blow a bubble and you observe it, you will see that there are colours in there. When the light falls on the bubble, at one point, there’s only one light ray and we need multiple light rays to get interference. So, what happens when it hits the bubble surface? A portion of the light is bounced off the surface of the bubble or the soap film. So, it’s gonna reflect right back on top of itself. When the reflected ray and the other ray that continues in the soap film overlap, our eyes experience interference. The interference might be constructive or destructive and we might see different colours.
Hence we can now say that interference is responsible for the colours appearing on a thin soap film.

The correct option is (A).

Note: The colours seen in a bubble depend on the film thickness. A bubble becomes thinner and thinner as it evaporates and dries out, before finally popping. As the surface film of the bubble becomes increasingly thinner, a change in overall colour can be seen. Thick walls block out longer wavelengths in the red visibility range. As the bubble gets increasingly thin, yellow wavelengths are blocked out. As it gets even thinner, the green light is lost. Further than this point, even short wavelengths of the blue visibility range are lost.