Why do bubbles show rainbow colours?

   Bubbles are balls of thin soap films. They show rainbow colours because the light falling on them undergoes what is known as interference. Light travels in the form of waves which have troughs and peaks at regular intervals. For instance, one wavelength of a light-wave has one trough and one peak. When light falls on a soap bubble it is reflected from both the upper and lower surfaces of the film. Thus, there are two waves of reflected light emerging out of surfaces of the soap bubble. If the distance between the two surfaces of the film is less than one wavelength of light, the two reflected waves are so close that interference occurs. The peaks of the two waves may add up and become a bigger peak. Or the trough of one wave may fall with the peak of the other thus cancelling its effect. As the seven component colours of white light have different wavelengths, the adding or cancelling effect of interference is not the same on all of them. In other words, some colours become brighter and some duller. Thus a viewer sees a mixture of rainbow colours instead of the white light emerge from the soap bubble.