Learn in the textbookOptical Disc Storage Media — CD, DVD, Blu-ray
DefinitionA CD (Compact Disc) is a 12cm optical disc (光ディスク) whose data is read by shining a laser beam onto the recording surface of the spinning disc and detecting changes in the reflected light.
ExplanationOn a read-only CD, tiny dents called pits run in a spiral across the recording surface, and the difference in reflection where a pit is or is not present encodes the data. It holds about 650 to 700MB, less than the later DVD and Blu-ray Disc. It first spread as a music format and was later adopted for storing computer data.
AnalogyLike a fingertip running over Braille, it uses light to trace the fine bumps on the disc surface.
Memory hookThe name comes from Compact Disc: it was far smaller (compact) than the vinyl records that came before it.