Reading Information from the CD is done by illuminating it with a focused beam from a diode laser.
The diode laser radiation is focused by an optical system into a small spot on the surface of the CD.
The radiation is reflected back to a detector from both the upper metalic surface, and the bottom of the pit.

One of the first designs of the CD was by laser light transmitted through small holes in the CD, and detected by an array of photo-electric detectors on the other side of the disk.

The CD today is based on the reflection of the laser light from the metal coating on the CD. The detector is mounted on the same side of the CD as the laser (as seen in figure 9.9).