A compact disc is a round, flat piece of plastic that is used to store and play back information. That information may include music, pictures, or computer programs and games. Compact discs are often called CDs. A type of CD used to store movies is called a digital videodisc, or DVD.

How a CD Works

A CD is about ¹/₂₀ inch (1.2 millimeters) thick and 4 ³/₄ inches (12 centimeters) across. On one side of the disc is a thin layer of metal. The metallic layer has very tiny pits, or holes. These pits are so small that they can be seen only with a microscope. The information is stored in these pits. The pits are arranged in a spiral, or winding path. The spiral starts in the center of the disc and winds around until it reaches the rim.

A CD is played by putting it into a CD player or a computer. The player or computer spins the CD. A laser, or strong beam of light, shines on the CD while it spins. The laser beam reflects off the metallic layer of the CD. The shape of the pits in the metal creates a pattern of reflected light. The CD player or computer reads this pattern of light and turns it into a form that a person can see or hear, or both.

History

Compact discs that stored sound were introduced in 1982. By the mid-1980s new discs called CD-ROMs could store pictures and computer programs. DVDs appeared in 1995.

Before CDs were invented, music and movies could only be stored on vinyl records, movie film, or cassettes containing magnetic tape. CDs are an improvement over those ways of storing information. The sound quality on a CD is better than on cassettes or records. CDs also last longer than film, magnetic tape, and vinyl.