# Why do we see rainbow colors when we look at the back of a CD?« Back to Questions List

Imagine a thin film of oil floating on water. When light falls on it, most of it travels through water (Ray ‘A’) while some is reflected off the top (Ray ‘B’) and bottom layers (Ray ‘C’) of the oil film. The light reflected from the bottom layer of film travels a little more distance than the light reflected from the top layer of the film.

Constructive Interference

Let us try to understand the concept of interference. When two waves are at the same place at the same time, the resulting wave is the addition of the two waves. This is called constructive interference. The amplitude of resulting wave would be more than the original wave.

Destructive Interference

Under similar conditions, when we add up two waves with first being up and second being down at the same time, the two waves would cancel each other. This is called destructive interference.

Coming back to our case, one light wave that hits the oil film gives back two waves (Rays A and B) and these two waves then interfere. This is called interference through reflection.

Now the thickness of the oil film is very important. When the difference in the path length between the observer and the two waves is in order of wavelength of visible light, then constructive interference occurs. When the difference in path length is only half the wavelength, then destructive interference  occurs.

The spots where the thickness of the film produces destructive interference will appear dark, while the spots where there is constructive interference will appear bright. This produces interesting patterns of light and dark.

In the case of a CD, the surface is mirrored and the angle at which you hold the CD controls the path difference. Rainbow colors are produced by constructive and destructive interference  of the light hitting the CD.

This effect can also be seen on a shiny colorful wrapping paper.

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