Memory and Storage

Hardware · 4 question types

uses a laser to read (and sometimes burn) microscopic pits and lands on a reflective disc to represent binary data.

How a CD, DVD or Blu-ray works

The disc is a thin layer of reflective metal sandwiched in plastic.
Data is recorded as a long spiral track of pits (tiny depressions) and lands (the flat regions between pits).
To read, a laser shines on the spinning disc; the reflection is bright over a land and dimmer over the transition between pit and land.
A sensor detects the changes and the drive translates them into 1s and 0s.
On writable discs (CD-R, DVD-R, BD-R), the laser also burns new pits into a dye layer, but this can normally only be done once. Rewritable formats (CD-RW, DVD-RW) use a special phase-change material that can be erased and rewritten many times.

Side view of an optical disc surface showing pits and lands and the stream of binary 1s and 0s they represent — Source: Secondary Storage by Save My Exams

Capacities

Disc	Capacity
CD	~700 MB
DVD	~4.7 GB (single layer)
Blu-ray	~25 GB (single layer), 50 GB (dual layer), up to 100+ GB (XL)

Advantages and disadvantages

Advantages	Disadvantages
Very cheap per disc	Very small capacity compared to HDD or SSD
Small and lightweight; easy to post or carry	Slow read/write speed
Discs are removable, so they can be archived	Prone to scratches and surface damage
No physical contact between laser and disc surface	Modern laptops often have no optical drive at all

Optical storage is now mostly used for legacy distribution (older software, music albums, films on Blu-ray) and archival: a properly-stored disc can outlast many digital drives.