Optical external memory uses a laser to scan a spinning reflective disk in which the presence or absence of nonreflective pits in the disk indicates 1s or 0s. This is the same technology employed in the audio CD. Because its contents are permanently stored on it when it is manufactured, it is known as compact disc-read only memory (CD-ROM). A variation on the CD, called compact disc-recordable (CD-R), uses a dye that turns dark when a stronger laser beam strikes it, and can thus have information written permanently on it by a computer.
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