Optical storage. The most popular forms of optical storage are the compact
disk (CD), which can hold about 700 megabytes of data and has a playtime of
about 80 minutes, and the digital video disk (DVD), which can hold 4.7 or 8.5
gigabytes of data per side of the disk (or up to 17 gigabytes on a two-sided
disk). The expression digital versatile disk is also used in place of digital
video disk, since DVDs can hold any digital data, not just video data. Data
are stored optically on a disk, and are read by a laser. A higher capacity
format called Blu-ray DVD can store 27 gigabytes per layer, or 54 gigabytes in
a double-layer disk.