JN>  JB> CD cross-section   (not to scale)

JN>  JB>     "label side"
JN>  JB> ---------------------  thin plastic
JN>  JB> ********************** data layer
JN>  JB> ---------------------
JN>  JB> |                     |
JN>  JB> |                     | thick plastic
JN>  JB> |                     |
JN>  JB> |                     |
JN>  JB> -----------------------
JN>  JB>       "data side"


JN>  You're saying there is data on both sides of a CD?

JN>  Including the back side which rests on the bottom of the cases
JN>  and which most everyone grabs to remove a CD from the case?

JN>  Are there 2 beams?  One to read each side at the same time?
JN>  (Learn something new everyday!)  

Nope, just one.
Look at a normal mirror, mirror on the wall...
A clear piece of glass, coated on one side with silver, then recoated
with paint to protect the silver from scratches. You're not seeing your
reflection from the surface of the glass, you're seeing the light that
passed through the glass, bounced off the silver coating, and made it
back to your eyes.

CDs work the same... the laser beam shoots through the clear plastic
side, bounces off the reflective "data layer", and back out the same
way it got in. If there's a "pit" in the data layer, the laser doesn't
reflect. If it's still smooth, it does reflect. There's the 1's and 0's
your computer wants to see.

If you scratch the data (label) side, or pull a chunk off with a
too-sticky paper label, or use a pen that disolves or leaches through
the label/data layer, you'll get holes or pits where there used to be
shiny stuff, and the CD is now a Frisbee(tm).

  * SLMR 2.1a *

--- Maximus 3.01