John Campbell asked:
How do re-writable CDs and DVDs work? We know that when you burn a CD you are burning tiny pits into CDs and DVDs to make digital recordings of sounds and images but how can you then undo that so you can rewrite them hundreds of times?
Dave - Okay. A normal CD works by just having a sheet of aluminium with lots sort of pits in it. You make the pits by having a glass. You etch a glass thing which you push into the sheet of aluminium, that makes the bumps in the aluminium and then shine a laser on it. Then they get reflected differently from the pits and tops and bottom 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. Itís encased in polycarbonate and so itís nice and protected. A recordable CD works by having the same polycarbonate disc. They have a layer of dye over the top. This dye is sensitive to light and we have a shiny thing behind it. Thereís various different kinds of dye with different - some little better than others and survive a long time thatís why you have different colours of CDs and CD-Rs and so it changes colour.
Chris Smith - 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 Smith - And then when you come along with another laser you can what, reset the dye to itís original colour which overwrites theÖ
Dave - I think they then hit it with a different pair of laser which heats it up to a different temperature which then re-sets it and then it cools down slowly and so it zeros everything and then you can come along with the second type analyser and we rewrite it.