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Like gramophone records, the information on optical discs are recorded on a spiral track. However, with a CD the laser starts reading the disc from the inside ring (table of contents) and ends up on the outside. When play back starts, a laser beam shines on the ridges and lands on the data membrane layer. If you look at the image on the right you can see the data layer moving in grey.
During playback, the number of revolutions of the disc decreases from 500 to 200 rpm (revolutions per minute) to maintain a constant scanning speed. The disc data is converted into electrical pulses (the bit stream) by reflections of the laser beam from a photoelectric cell.