I've always been intrigued by the dividing line between the written and unwritten areas on a CD-R. All CD-R formats begin writing in a spiral track starting at the inner radius of the disc and proceeds to the outer radius, so the inner area is written data, while the outer area is still blank. The inner area usually looks lighter in color.
I first placed the CD-R on the microscope stage, bottom-up. This worked at "medium power" 10x, but I could not focus at 40x because the thickness of the CD itself did not permit focusing the 40x objective. Instead, I scratched off a little of the protective coating on the CD top surface, then inspected it from the top.
This is a very old Memorex CD-R. Green-blue dye with a gold top-layer. I lit the CD-R from below with the microscope's built-in light. The image was pretty dark, but by setting the camera exposure to 3 sec, the final image looks great.
Here's a shot of the dividing line between the data area (upper half) of the disk and the blank unused portion of the disc (lower half). The length scale is approximate.
Here is a %100 crop from my camera. The length scale is approximate and computed by dividing the frame height of my camera (13mm) by the microscope objective's magnification (40x), then scaling in GIMP to make the bar an appropriate size for the crop. I checked Wikipedia, and it appears the track spacing for a CD-ROM is 1.6um, so I wasn't too far off.