Physical Organization of CD-ROM II
When we read the stamped copy of the disc, we focus a beam of laser light on the track as it moves under the optical pickup. The pits scatter the light, but the lands reflect most of it back to the pickup. This alternating pattern of high- and low-intensity reflected light is the signal used to reconstruct the original digital information.
1’s are represented by the transition from pit to land and back again. 0’s are represented by the amount of time between transitions. The longer between transitions, the more 0s we have.