HI Lou,

   Thank you for your information. But, my friend said he scanned the CDR
before and after writting. He could only see those tracks after writing. So,
what's happen? Do you think those tracks are only appeared in certain area?
That's why he may scan different areas and conclude his observation?

 -Hoyin


Ho Yin,
  CD-R disks like the one you have use an organic dye to do the actual
recording.  Data is recorded by exposing teh dye to a high power laser pulse
which alters the dye chemically, changing its reflectivity in a small spot.
This reflectivity change is what's being read by the player.  Similarly,
DVD-RW, uses a phase change alloy rather than a dye.  The tracks your friend
has found wiht AFM are not the data.  They are the servo pattern that is
read by the laser to determine where on the disc it is and to help it stay
on track.  These are embossed either in a resin or in the polycarobnate
itself before the coatings (dye, reflective layer - Aluminum or other, etc)
are applied.  Try google searching "phase change media" or something imilar
to learn more.

 -Lou