How do they work

  A schematic diagram of a design for an optical disc drive is shown in the picture.

• The optical disc has one spiral track running from the inner extreme of the surface to the
  outer edge.
  
• During operation, the disc spins.
  
• Simultaneously the laser moves across ensuring that it is continuously focused on the
  spiral track.
• The track on the surface of the disc has what is referred to as ‘pits’ and ‘lands’.
  
• The laser beam is reflected from the surface of the disc.
  
• The difference between the reflection from a pit compared to that from land can be
  detected.
  
• This difference in the intensity of the light the detector receives can be interpreted as
  either a 1 or a 0 to allow a binary code to be read from the disc.

For CD-RW and DVD-RW technologies, the reflective surface is a special alloy material. When
data is being written to the disc (the ‘burn’ process) the heat generated by the absorption
of the laser light changes the material to liquid form. Depending on the intensity of the laser
light the material reverts to either a crystalline or an amorphous solid form when it cools.
When the disc is read, the laser light is reflected from the crystalline solid but not from the
amorphous solid, allowing the coding of a 1 or 0.