Compact discs. Assortment of compact discs (CD's). Each disc stores audio data such as music, as a series of fine depressions, representing a digitized 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, which follows the depressions exactly. This layer is covered by another layer of transparent plastic, which prevents dust and scratches from affecting the sound quality.