...
The elongated bumps that make up the track are each 0.5 µm wide, a minimum of 0.83 µm long and 125 nm high. They look something like this: 
You will often read about "pits" on a CD instead of bumps. They appear as pits on the aluminum side, but on the side the laser reads from, they are bumps.
...
A CD is read by focusing a 780 nm wavelength- infrared level, semiconductor laser through the bottom of the polycarbonate layer. The change in height between pits and lands results in a difference in the way the light is reflected. By measuring the intensity change with a photo-diode, the data can be read from the disc. 
DVD - Phat
Bluray - Thai