Compact discs (CDs), 2004.

UNITED KINGDOM - FEBRUARY 13: Compact audio discs were developed by Philips and Sony in the 1980s. The disc consists of a clear layer of polycarbonate plastic laid on top of a reflective aluminium surface. Data is magnetically recorded and stored on the disc digitally in binary code. The �1s� are represented by small pits in the plastic, while the �0s� are the remaining smooth plastic surface. When the disc is played, a laser scans its surface with the effect that only the �0� areas reflect the beam back. These reflected pulses are picked up by a photodetector, which converts them into a digital electrical signal. By the early 1990s CDs had become the prevalent medium for audio recordings and digital storage. (Photo by SSPL/Getty Images)