How does a laser make musical sound off of a CD?

Music is stored on a compact disk (CD) as digital data. On the bottom side of a CD is a thin metal sheet. Tiny round depressions called pits, which represent sound, fill its spiral track. A beam of laser light is used to change a CD’s pitted track back into sound. Unlike ordinary light, which spreads out in all directions once it leaves its source, laser light can be focused with great accuracy. It moves along the track of a CD, and sensors detect the pattern of shiny flat parts (which reflect light back) and pits (which do not). These on-and-off flashes of reflected light turn into electrical signals. A computer in a compact disc player, which has an enormous memory that stores every possible combination of on-and-off patterns, converts the signals to musical notes with different pitches and volumes in the player’s speakers. And then the music plays!


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