How do re-writable CDs and DVDs work?

Dave Ansell took on this question...

Dave - Okay. A normal CD works by just having a sheet of aluminium with lots of sort of "pits" in it. You make the pits by having a glass. You etch a glass object which you push into the sheet of aluminium and that makes the bumps in the aluminium; you then shine a laser on it. They laser light gets reflected differently from the tops and bottoms of the pits and you can read that information and then that information gets turned into sound and you can hear it.

The sheet of aluminium is a shiny thing. It's encased in polycarbonate and so it's nice and protected. A recordable CD works by having the same polycarbonate disc but they instead have a layer of dye over the top. This dye is sensitive to light and a shiny surface is placed behind it. There are various different kinds of dye; some are a little better than others and survive a longer time: that's why you have different colours of CDs and CD-Rs, and so it changes colour...

Chris - Oh, so when you zap it with a laser, rather than burning hole in it, what it actually does is changes the dye configuration so that what the reader is looking for is a dark spot rather than a hole.

Dave - Yes, and the dye changes colour.

Chris - And then when you come along with another laser you can what, reset the dye to it's original colour which overwrites the impression?

Dave - I think they then hit it with a different pair of lasers, which heats it up to a different temperature which then resets it; then it cools down slowly and so it zeros everything. Then you can come along with the second type of laser and rewrite it.