Like old records, information on optical discs is recorded in spirals. CDs start reading from the inside ring and reads to the outside. A laser beam shines on ridges of the CD and ends up on a data layer. The data layer moves. During playback, the number of revolutions decrease from 500 to 200rpm to keep a steady scanning speed. The data on the disc is converted into electrical pulses, or bit stream, using reflections of the laser beam that comes from a photo electric cell. The laser strikes when it is reflected and reads the disc. Optical scanning is the way. The difference in the height of the surfaces between ridges is a quarter of a wavelength.