Sometimes when I leave a disk unlabeled, I have to look at
the underside to see. You can tell because the CD has a
darker inner ring where the data is burned.
While this data isn't boldly darker, it's visible. The color
you see is the composite of all the 1s and 0s in the files
you've
burned. The 0s are the natural state of the disc;
you add 1s as you go to make an actual file. Since data is
usually only about 20% 1s, it stands to reason the darkest
possible shade of data on a CD is 5x darker than what you
normally see.
By perfectly syncing the linear stream of data to the spiral
of the disc, it is theoretically possible to line it up so that
you could create a coherent image, and possibly even a
photograph.
While not particularly useful, I propose a software that
creates ISO files that, when burned to a standard CD,
would show a line drawing, block drawing, or even a
photograph.