The polycarbonate substrate is coated with a thin layer of metal, usually aluminum or gold. This layer reflects the laser beam, and it is this reflection that is detected by the CD player.
The data on a CD is stored in the form of pits and lands. Pits are small depressions in the metal layer, and lands are the areas between the pits. The pits and lands are arranged in a spiral track that runs from the center of the CD to the outer edge.
When the CD player reads data, the laser beam scans the track and detects the pits and lands. The pits cause the laser beam to scatter, while the lands reflect the beam directly back to the detector. The detector converts the reflected light into electrical signals, which are then processed to extract the data from the CD.