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Word Count: 483
How a CD works Compact disc CD players have increasingly replaced the phonograph and the cassette tape They offer a clear crisp sound faint background noise and have the additional advantage of longer life In addition to their audio content some compact discs contain digital graphics that can be displayed on a television screen To understand how a CD works one must understand what a CD consists of A CD is about 12 millimeters thick Most of the CD consists of an injection-molded piece of clear polycarbonate plastic During manufacturing a high precision laser beam is used to burn microscopic pits arranged as a very long spiral track of data in the thin layer of a disk Once the clear piece of polycarbonate is formed a thin reflective aluminum layer is placed onto the top of the disc Then a thin acrylic layer is sprayed over the aluminum to protect it Finally a label may be printed onto the acrylic Compact discs are never in physical contact with any pickup devise Instead pits embedded in the surface of the disc are read by a laser beam of light A CD player contains a low power laser and high precision lenses and mirrors A servomotor positions the lenses and mirrors to attract on the disc The laser directs a narrow beam of light onto tracks of the spinning disc Along a track sections with pits scatter the light differently from ridges or sections without pits The sequence of the sections represents the sound information The laser starts reading the disc from the inside and ends on the outside While a CD is spinning a laser beam shines on the pits and ridges When the beam strikes a ridge the beam is reflected onto a photoelectric cell or photodetector and gives off a current When the laser beam shines on a pit only half of the light hits the surface The other half
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