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why is rainbow formed in CDs?

Similar to water drops in falling rain, the CD separates white light into all the colors that make it up. The colors  reflecting from a CD are interference colors as  on a soap bubble or an oil slick.

The light as as being made up of waves-like the waves in the ocean. When light waves reflect off the ridges in the  CD, they overlap and interfere with each other. Sometimes the waves add together, making certain colors brighter, and sometimes they cancel each other, taking certain colors away.This creates a constructive and destructive interference of the light.  The compact disc is made up of very thin film.If the film thickness is on the order of the wavelength of light, then interference can occur easily.


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