How a CD turns reflected light into data
A CD’s data is arranged along one long spiral track, much like a groove winding from the center outward. The familiar “pits” and flat areas, called lands, are molded into a thin layer beneath the disc’s clear plastic surface and covered by a reflective coating. The player reads them from below, through the plastic.
The laser’s light reflects differently from a pit than from a land. A pit is shallow enough to shift the reflected light’s phase—roughly, the timing of its light wave—relative to light from a neighboring land. When those waves meet, they interfere, changing how much light reaches a sensor. The player turns that change into an electrical signal.
The important information is in the transitions
A pit does not simply mean “one,” with a land meaning “zero.” Instead, the encoded data is represented by the pattern of changes between pits and lands. The player’s electronics detect those transitions and reconstruct a stream of bits, while also recovering timing so it can tell where one bit ends and the next begins.
That signal still needs processing before it becomes music or computer files. The disc’s encoding and error-correction information help the player recover data despite small scratches or dust, and the player uses servo mechanisms to keep the beam focused on the narrow spiral as the disc spins. The laser is not taking a picture of the disc; it is sensing tiny changes in reflected light and letting the electronics interpret their sequence.
