When we see at the surface of the CDs, we see a pattern of colours just like the rainbow. This formation of patterns of color also occurs in soap bubbles and in thin layer of oil over water. The phenomenon which governs this is the interference of light. Interference of light is the phenomenon which occurs when two or more light waves travel in the same medium. Interference is classified as constructive and destructive according to direction of the interfering waves. If the two waves are traveling in the same direction, then it is called as constructive interference and if the direction is opposite then it is termed as destructive. Constructive interference leads to the summing of the wave amplitudes and in destructive it is just the opposite. When white light strikes on the reflecting part of the CD or the thin films, some part of the light is refracted and a small part of the incoming light is reflected back. The reflected waves consist of two components: one is the wave reflected from the upper part of the film of CD and another one is from the bottom of the thin film in CDs. As there is a path difference between the two reflected light waves, they can now be considered as different light waves traveling in the same direction which is the main requisite for interference. Due to interference, pattern of colors like rainbow is formed.