I have used a test CD-R for that experiment (just out of curiosity):
I have stuck transparent clear tape on a portion of a disc.
The result is that the disc is entirely unreadable on that portion, as if the tape were black instead of transparent.
I know, this is not a situation that would encounter in real life, but I am still curious:
How come a thin layer of clear tape (that is nearly invisible with the naked eye) is enough to make the disc entirely unreadable (until removed)?