When Casey looks at a CD or DVD she notices that there is a rainbow pattern that is created by the light. She became curious and shone light directly at it and a pattern was created on the wall. What is the cause of this pattern?

Interference

Explanation:

A CD is made up of thousands of pits arranged in the form of spiral tracks. These repeated patterns of pits form a thin film.

This film has the thickness of the order of the wavelength of the light which causes the interference of the waves of the light waves. The incident ray of light gets reflected from the top layer of the film partially and then after striking the bottom layer of the film creating a phase difference responsible for the constructive and destructive interference.

diffraction