CDs are easily scratched, and should never be cleaned with just any cloth. CDs should be cleaned radially: not along the grooves, but at right angles to the direction of the grooves. If a smear, however small, should remain on the CD, running along the direction of the grooves, much information would be lost.

Does a CD have grooves in it?

The polycarbonate disc contains a spiral groove, called the pregroove because it is molded in before data are written to the disc; it guides the laser beam upon writing and reading information.

How many lines does a CD have?

On a CD the track pitch is 1.6 microns or 625 lines per mm. A 3.95GB DVD-R has a track pitch of 0.8 microns or 1,250 lines per mm. A 4.7GB DVD-R has a track pitch of 0.74 microns or 1,351 lines per mm. So a DVD-R acts as a diffraction grating with 1,351 lines per mm and a CD-R woud have about half that.