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Compact disc. Photograph of an audio compact disc. The CD stores music as a series of fine depressions, representing a digitised musical signal capable of being read by a laser. To reflect the laser light, the music layer is coated with a fine film of metal, normally aluminium, which follows the depressions exactly. The reflective surface is covered by a thick layer of transparent plastic which prevents dust scratches from affecting the sound quality. A standard audio CD can store about 80 minutes of music. (KEYSTONE/SCIENCE PHOTO LIBRARY/DAMIEN LOVEGROVE)