Also known as: CD, DVD, Blu-ray
An optical disc stores data as microscopic pits and flat lands on a reflective spiral track, 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 from one long spiral track. Successive generations shortened the laser’s wavelength to pack data more tightly: the infrared CD, the red-laser DVD, and the blue-violet Blu-ray, whose shorter beam focuses to a smaller spot and reads finer pits. Discs come as read-only pressed media, write-once recordable, and rewritable types, and data is laid out under standard file systems so any compatible drive can read it.
Optical storage is cheap to press in volume, physically robust, and needs no power to hold data, but it is slow to read and write compared with flash, and mechanical drives to spin it are increasingly rare on new machines.
DVD vs Blu-ray
The jump from red to blue-violet lasers is what multiplied capacity across generations:
| Format | Laser | Wavelength | Capacity (single layer) |
|---|---|---|---|
| CD | Infrared | 780 nm | ~700 MB |
| DVD | Red | 650 nm | 4.7 GB |
| Blu-ray | Blue-violet | 405 nm | 25 GB |
Shorter wavelengths focus to a smaller spot, so pits shrink and the spiral packs more turns into the same disc.
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. ↩