Storage disk surface. Light micrograph of the surface of a magneto-optical disk (MO disk). The disk stores data as bits (dark dots). Eight bits forms a byte. The data is written on to the disc by a laser and a magnet. The laser heats a bit to between 180-200 degrees Celsius, the point at which molecules can be realigned when subjected to a magnetic field, and then the magnet changes the bit's polarity. The disk is read by detecting light reflected off the bit from a lower-powered laser. MO disks have a 3-year shelf life and can be rewritten up to a million times. Magnification x130 when printed at 10 centimetres wide.