How do CD-RWs or rewritable CDs work?

The way a Cd works is by storing data in a digital format (0, 1), in a cd this is archived by creating thousands of bumps on the surface of a cd (the space left behing between bumps will look like pits)The way this is done is by making the surface of the CD a mirror, and the bumps disrupt the mirror's perfect surface thus making it less reflective.

The laser that reads the CD can detect the difference between a perfect mirror and an imperfect part caused by a bump because of the difference in reflection that is getting back. By interpreting the perfect mirror as a "1" and the bump or blurry part as a"0" digital information is store on a CD. The bumps on a CD are molded into the plastic when it is manufactured, so they are permanent.

To create a writeable CD (CD-R), you need to modify the surface of a CD so you can burn data onto it, turning it into a CD-R. There are no bumps on a CD-R. A clear dye layer covers the CD's mirror. A write laser heats up the dye layer enough to make it opaque. The read laser in a CD player senses the difference between clear dye and opaque dye the same way it senses bumps -- it picks up on the difference in reflectivity.

To create a rewriteable CD (CD-RW), you need a dye layer that can be changed back and forth between opaque and transparent. To do this a CD-RW's uses a special material. The material has the property that it can change its transparency depending on temperature. Heated to one temperature, the material cools to a transparent state; heated to another temperature, it cools to a cloudy state. By changing the power (and therefore the temperature) of the writing laser, the data on the CD can be changed, or "rewritten."

A CD-R can be read by just about any CD player. CD-RW discs are not so versatile -- lots of older CD players cannot read them.