Playing music by laser beam
A compact disc is only 4/4in (120mm) across, but it holds 3 miles (5km) of playing track and plays for about an hour. CDs are played on one side only, and do not get scratched during play, or wear out, because no needle or stylus touches the surface. Instead a light beam from a low-powered laser reads the disc from the underside, and interprets microscopic pits and flat areas on the playing track that spirals out from the centre of the disc. Just as the Morse code operates by dots and dashes, so the pits and flats are a digital code, waiting to be interpreted into sound.
Digital codes work by forming patterns using just two digits - 0 and l. From these two, a code can be compiled that represents an infinite richness of patterns and sounds. Any sound within the range of the human ear - from a single drumbeat to a full orchestra in crescendo - can be faithfully reproduced upon decoding.
As the laser beam scans the rotating disc, the various pits and flats affect the beam's reflection, which falls on a light-sensing device called a photodiode. This device converts the information that falls on it into electrical signals. These signals are decoded electronically into variable electric current, and then amplified and fed to the loudspeakers, which reproduce the sound waves that led to the creation of the pits and flats in the first place.