Optical disc
Storage media read with a laser. Data sits as pits and lands along a spiral track beneath a protective polycarbonate layer; the difference in reflection becomes ones and zeroes.
Capacity follows wavelength: CD uses infrared light at 780 nm and holds 700 MB, DVD red at 650 nm and 4.7 GB per layer, Blu-ray blue-violet at 405 nm and 25 GB per layer. A shorter wavelength gives a smaller focal point, hence tighter tracks. Recordable discs use a dye layer that the laser darkens instead of physical pits, which is worse for long-term storage — "disc rot" renders cheap CD-Rs unreadable after five to ten years. M-DISC burns into an inorganic layer instead and is marketed as lasting a century. The format is on its way out for consumers but survives in archival use, precisely because a disc cannot be encrypted by ransomware.