Optical discs. Compact discs (CDs) and digital versatile discs (DVDs) are examples of optical discs. An optical disc digitally stores music and other data as tiny depressions in a plastic disc. This disc is coated with a fine film of metal and then by another layer of plastic. The depressions form a spiral track moving from the centre of the disc to the outer edge. The data track is about 0.5 micrometres wide, and would stretch out for nearly 5 kilometres if unravelled. The information is read by a laser light that reflects from the metal layer, allowing the original data to be reconstructed.
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