This concerns the cause of the diffraction patterns seen in the reflected
light from CD's. Looking at an audio CD with an incondescent bulb as a
light source, the continuous spectrum extends away from the light
beginning with blue and evolving into red. This is consistent with
looking through a diffraction grating in that the blue lines are nearest
the central peak and the reds are farther from the central peak. This
implies to me that there are very closely spaced, concentric rings of
varying reflectivity. However, I looked at one with a fairly high
powered, stereo microscope and I don't see a ring pattern of anything. I
think that I remember hearing that there are pits in the reflecting
surface which are a binary representation of the information, but I
didn't see what I expected to see. I did see lots of tiny, whitish or
grayish dots, but they didn't seem to have much of a pattern. There is
an outer region (about 3/8 " wide) with some appearance of concentric
rings and inside this region the dots don't seem to be in a pattern.
Since the interference pattern extends completely across the disk, I am
confused.
Does anyone KNOW what the pattern actually is? The dots I saw are so
tiny, I can't believe that a laser beam is focused to such a small area.
Is there a mask of some sort between the disk and the "read head"? How
is the sequence of material (music) distributed on the disk? How are the
"tracks" addressed? Is there sectoring as on magnetic disks? Does
anyone have a good reference for this information?
Is there any fundamental difference between computer CD-ROMs and audio CDs?