Did you know you can actually see where the data is on a CD? It's true!
Take a look at the first image: there's a more reflective part on the outside, and a slightly less reflective part on the inside. The more reflective part is empty, while the other part has the audio. It means you can actually gauge by eye how much data's on a CD. I've done that plenty of times, to guess how long a disc will take to rip.
You can also see gaps between sections of data. That second CD is a multisession disc - a somewhat rare technique used to separate out data more cleanly than just mixing data and audio within a single normal CD session. This is an audio CD with bonus digital contents. The part towards the centre of the disc is music, then a small gap between sessions that contains nothing, followed by more data that has the computer data in the second session.
And now you know! You'll never use this fact, but at least it's there.
