Describe what happens inside a CD
player when you listen to an audio CD.

Inside your CD player, there is a miniature laser beam (called a semiconductor diode laser) and a small photoelectric cell (an electronic light detector). When you press play, an electric motor (not shown in this diagram) makes the disc rotate at high speed (up to 500rpm). The laser beam switches on and scans along a track, with the photocell, from the center of the CD to the outside (in the opposite way to an LP record). The motor slows the disc down gradually as the laser/photocell scans from the center to the outside of the disc (as the track number increases, in other words). Otherwise, as the distance from the center increased, the actual surface of the disk would be moving faster and faster past the laser and photocell, so there would be more and more information to be read in the same amount of time.

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