Coloured scanning electron micrograph (SEM) of Compact disc surface (CD, CD-ROM). A CD is a plastic disc that is etched or pressed with a series of fine depressions representing the digitized audio or musical signal. A very thin layer of metal is used to coat this surface (including the depressions) so that the laser light can be reflected. Another thin layer of plastic overlies the metal surface for protection. Magnification: x2, 250 when shortest axis printed at 25 millimetres.