Optical storage: CD-ROM and DVD

Optical discs store data as tiny physical features along a spiral track on a reflective surface, read by a focused laser beam. The compact disc was first used for audio and later adapted for computer data as the CD-ROM, a read-only medium pressed with data at manufacture. As a laser scans the spinning disc, transitions between the pits and the flat lands change how light reflects back to a sensor, and this varying reflection is decoded into the stored bits.

Beyond pressed discs, recordable formats let users write their own data. A write-once CD-R uses a dye layer that a laser alters permanently, while a rewritable CD-RW uses a material whose reflective state can be changed back and forth, allowing the disc to be erased and reused. The DVD applies the same optical principles with a more tightly packed track and a finer laser spot, so a DVD holds considerably more data than a CD, which is why it became the medium for video and large software. Related formats included the Video CD and the interactive CD-I.

To ensure that discs written on one device can be read on another, data is arranged according to agreed file system standards; the widely used ISO 9660 standard defines a common layout for CD-ROMs so they are portable across systems. Like other storage, optical discs organise data into structures the operating system can navigate, and a data disc can carry the boot information needed to start a computer, which is why installation and recovery software was often distributed on CD or DVD.

Frequently asked questions

How does a CD-ROM store data?

Data is held as pits and lands along a spiral track; a laser scans the spinning disc and the changing reflection at each transition is decoded back into the stored bits.

What is the difference between CD-R and CD-RW?

A CD-R can be written once using a dye layer that is permanently altered, while a CD-RW uses a material that can be switched back and forth, so the disc can be erased and rewritten.

Why does a DVD hold more than a CD?

A DVD packs its track more tightly and uses a finer laser spot, allowing far more data in the same physical size, which made it suitable for video and large programs.