Also known as: CD, DVD, Blu-ray
An optical disc stores data as microscopic pits and flat lands on a reflective surface, read by a focused laser.1
Overview
A laser shines on the spinning disc and a photodetector senses how the reflected light changes between pits and lands, recovering the bit stream. Successive generations shortened the laser’s wavelength to pack data more tightly: the infrared CD (~700 MB), the red-laser DVD (4.7 GB and up), and the blue-violet Blu-ray (25 GB per layer and beyond). Discs come as read-only pressed media, write-once recordable, and rewritable types. Data is laid out under standard file systems so any drive can read it.
Where it fits
Optical discs were the main way software, music, and films were distributed before broadband and flash storage took over, and they remain useful for cheap, long-lived archival because a written disc needs no power and resists data rot if stored well. In the memory hierarchy they sit alongside magnetic tape at the slow, archival end — fine for keeping a frozen snapshot of GopherTrunk logs offline, but far too slow for live capture, where an SSD or HDD is required.
Sources
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Optical disc — Wikipedia, on CD/DVD/Blu-ray optical storage and how lasers read pits and lands. ↩