CD Basics: The Bumps

A CD has a long, spiraled data track. If you were to unwind this track, it would extend out 3.5 miles (5 km).

If you've read How CDs Work, you understand the basic idea of CD technology. CDs store music and other files in digital form -- that is, the information on the disc is represented by a series of 1s and 0s (see How Analog and Digital Recording Works for more information). In conventional CDs, these 1s and 0s are represented by millions of tiny bumps and flat areas on the disc's reflective surface. The bumps and flats are arranged in a continuous track that measures about 0.5 microns (millionths of a meter) across and 3.5 miles (5 km) long.

To read this information, the CD player passes a laser beam over the track. When the laser passes over a flat area in the track, the beam is reflected directly to an optical sensor on the laser assembly. The CD player interprets this as a 1. When the beam passes over a bump, the light is bounced away from the optical sensor. The CD player recognizes this as a 0.

A CD player guides a small laser along the CD's data track.
In conventional CDs, the flat areas, or lands, reflect the light back to the laser assembly; the bumps deflect the light so it does not bounce back.