Compact discs. Photograph of several audio compact discs (CD). 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.