Having thickness of mere 1.2 millimeters and diameter of 12 centimeters CD weighs only 28 grams but has capacity to store 74 minute of music recording or 783 megabytes data on its 5 kilometers long spiral track. Compact disc mainly comprises of three layers. The topmost layer is made of acrylic, followed by aluminum and the bottom layer of polycarbonate. This bottom layer is transparent, passing through which the laser beam ‘reads’ 0.5 micron broad pits recorded on the aluminum layer. These pits are actually indicative of binary digits 0s and 1s and each 0 and 1 in turn is indicative of the connected data (viz. audio, video).
When CD is inserted in the disc drive following sequence of events takes place: Laser beam strikes the aluminum layer of CD penetrating through the transparent polycarbonate layer and is reflected towards the photo detector situated beneath the CD. (This process takes place many times in a second.) Since the pit is not flat it does not reflect laser much whereas the flat surface reflects laser more.
Photo detector detects changes in the reflected beam and constantly passes on information to electronic circuit. It is the job of this circuit to convert the digital data into electronic signals. These are processed and decoded to form sound signals that can be amplified for reproductions on loudspeakers and visual signals that can be relayed on TV.
The experts have revolutionized the field of data storage by applying this simple principle in DVD in addition to CD. In a way, the credit for this revolution belongs to James Russell.
Compact Disc (Wikipedia)