Optical disc surface. Differential interference contrast (DIC) light micrograph of the surface of an optical disc. Compact discs (CDs) and digital versatile discs (DVDs) are examples of optical discs. An optical disc digitally stores data, such as music and films, as tiny depressions (green) in a plastic disc (orange). Each depression is either long or short, each representing a bit. Eight bits form a byte. CDs can hold millions of bytes of data. DVDs can hold billions of bytes of data. The data track is about 0.5 micrometres wide, and is several kilometres long. The information is read by a laser light that reflects from a metal layer, allowing the original data to be reconstructed. Magnification: x2250 when printed 10cm wide.