點解cd的背面反光會發出彩色的光

This is due to the phenomenon of "thin film interference".

Light incident onto a thin film will be reflected, part at the top and part at the bottom of the film. When these two reflected light rays interfere with each other, depending on the angle of incident and the thickness of the film, constructive interference occurs. A bright light band is thus observed.

Since white light (or sunlight) composes of seven colours. If white light is incident obliquely onto a thin film, different colours of the white light will form bright bands by constructive interference at some suitable angles determined by the wavelength of that coloured light. Hence, a colourful pattern is observed.

Compact disc ( CD) is made of a polycarbonate wafer which is coated with a metallic film, usually an aluminum alloy. The aluminum film is then covered by a plastic polycarbonate coating. The coatings are less than 100nm thick and each coating partially reflects and partially transmits incident light. Light rays reflected from different coating boundaries interfere with each other to produce the colorful patterns.

For detailed theory on thin film interference, please refer to the web-page below:

http://electron9.phys.utk.edu/phys136d/modules/m9/film.htm

Explanation of the coloured pattern on compact disc is given near the bottom of the web-page.

interference of the wavelength.