Computer artwork showing light emitted by a laser diode being focussed onto a compact disc by a small lens. The metal disc underneath is etched by a laser beam to produce the microscopic pits which carry the digital code representing sounds. The code is read by a laser reflecting the pits of the etched surface back through the lens onto a sensor in a CD player which converts the signal back into music. To reflect the laser light, the music layer is coated with a fine film of metal which follows the depressions exactly. Each depression is either long or short, each representing a bit. Eight bits form a byte. CDs can hold millions of bytes of data. DVDs can hold billions of bytes of data. The data track is about 0.5 micrometres wide, and is several kilometres long.