A compact disc is a form of optical disc that data is written onto and read by lasers. A layer of aluminum is placed on top of a thicker layer of polycarbonate, with the data written into the aluminum layer by creating pits and lands in the material.
Digital data is written to the aluminum layer in binary form, with the binary value of 0 represented as a pit, and the value of 1 represented as a land. As the laser passes over the CD, it hits the pits and lands. As the pits do not reflect the laser, the device reads as 0, while the lands reflect the laser, which the device recognizes as 1. CDs can store up to 700 megabytes of data, while other formats of optical disc, such as DVD and Blu-ray, can store larger amounts of data.
Data is written to a disc in a spiral from the inside of the disc towards the outside; as the disc spins, it reads the data as it is written, translating the binary data into digital audio. CD players slow down the spin of the disc as the laser reaches the outer edge of the disc to keep the rate of data read from the disc at a consistent speed from beginning to end.
Writable CDs use an organic dye layer between the aluminum and plastic. When struck by a writing laser, this creates the pits and lands that store the information onto the disc. Discs that are rewritable use a crystalline alloy instead of dye, which can be melted back into crystal form by a disc drive that supports burning discs.