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In response to the comment above that it is impossible to identify a manufactured CD by looking at the data side:

Not strictly true. On CDs with "Secret Tracks", it is in fact possible to identify the CD by looking at the pits and grooves.

Try this: Take your copy of "Nevermind", by Nirvana. (You do have one, don't you? :-) Unless its one of the earliest pressings, it should contain the secret track "Endless, Nameless" after Something in the Way. Look closely at the back of the CD, and about 5mm in from the outer edge, the surface becomes slightly darker, returning to the normal shade after another 4mm or so. This dark patch is the ten minutes or so of silence between E,N and SitW.

If you know the exact times of secret tracks, and what those times correspond to in terms of radius on a disc, it is possible to make an educated guess as to what the CD is by looking for the "dark ring(s)".

I can only speculate as to the reason for silence being darker than sound, but my guess would be something along the lines of there being less variety of pits and grooves, thereby causing less interference/diffraction of light reflected off the CD.