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Light micrograph of a compact disc showing the diffraction of light from the music grooves (left) & soundles grooves (green bands). The red paint is written track information. The colours are produced by interference between reflected rays of light from adjacent grooves. The disc is made of plastic, which is pressed with a series of fine depressions representing a digitised musical signal, which is read by laser. To reflect the laser light, the plastic is coated with a metal, which follows the depressions exactly. This layer (music layer) is covered by a layer of transparent plastic, which prevents dust & scratches from affecting the sound. Mag:X10 (at 35mm size). (KEYSTONE/SCIENCE PHOTO LIBRARY/DR JEREMY BURGESS.)