CD Basics
CDs are made of a transparent plastic disk with a hole in the middle. On top of the disk is a thin layer of aluminum. The plastic layer serves mostly to protect the aluminum layer where the actual data is written. That is why a scratch on the bottom of the disk is usually easy to repair, but a scratch on the top can easily ruin the CD.
CD Encoding
The underside of the Aluminum coating has a spiral track like on a record. This spiral track has either small pits in it in a manufactured CD, or small patches of dark ink in a computer-burned CD. All of these pieces of information together are assembled by the CD player into a song.
CD Player
The CD player has a turntable like a record player, which holds the CD and spins it very quickly. It also has a laser on a movable arm which follows the groove of the CD as it spins. The laser shines a light onto the CD, which bounces off the metal coating and into a light detector. When the laser hits a pit or a dark patch of ink, it lowers the level of light shined back. The light detector records tens of thousands of flickers of the light every second based on the tiny pits or patches of ink. It sends this information to a computer, which decodes it, turning the digital signal into sound waves.
Tags: light detector, like record, spiral track