A CD-ROM drive operates by using a laser to reflect light off the bottom of the disc. The reflected light is then read by a photo diode detector. The overall operation of a CD-ROM drives is as follows.

  1. The laser diode emits a low-energy infrared beam toward a reflecting mirror.
  2. The servo motor, on command from the microprocessor, positions the beam onto the correct track on the CD-ROM by moving the reflecting mirror.
  3. When the beam hits the disc, its refracted light is gathered and focused through the first lens beneath the platter, bounced off the mirror, and send toward the beam splitter.
  4. The beam splitter directs the returning laser light toward another focusing lens.
  5. The last lens directs the light beam to a photo detector that converts the light into electric impulses.
  6. These incoming pulses are decoded by the microprocessor and sent along to the host computer as data.