What is a CD?

A compact disc is a thin, circular disc of metal and plastic about 12cm (just over 4.5 inches) in diameter. It's actually made of three layers. Most of a CD is made from a tough, brittle plastic called polycarbonate . Sandwiched in the middle there is a thin layer of aluminum . Finally, on top of the aluminum, is a protective layer of plastic and lacquer . The first thing you notice about a CD is that it is shiny on one side and dull on the other. The dull side usually has a label on it telling you what's on the CD; the shiny side is the important part. It's shiny so that a laser beam can bounce off the disc and read the information stored on it. sony cfd s70 How CDs use optical laser technology Until CDs were invented, music was typically stored on vinyl (plastic) LP (long-playing) records and cassette tapes. LPs scratched easily, while tapes could stretch and distort and sometimes snapped or seized up entirely. Both ...