Quote Originally Posted by bah View Post
The data on a cd-r is stored in the foil thats stuck to the top of the disc (along with some protective/reflective layers). Its basically a bit of plastic, then a layer than can go from transparent to opaque when the burning laser hits it, then a reflective layer and some protective ones.

If the laser burns a spot, then when read back the reflective layer cannot be seen in that spot.

The plastic just makes the thing rigid, the laser focuses right through it.
I always thought it was, as when I first had CD-R's (back in 1999) I accidently scraped off some of the foil (writing on it with the wrong type of pen) and revealed the clear plastic and wondered why it didn't work thereafter. I've learnt since, that in caring for compact disc's it's just as important to look after the top as it is to look after the underside and prevent scratches.