Answer
Interference of light waves accounts for the bands.
Work Step by Step
The light bands are regions of constructive interference, when the wave reflecting off the top surface of the lower piece of glass is in phase with the wave reflecting off the bottom surface of the upper piece of glass.
The dark bands are regions of destructive interference, when the wave reflecting off the top surface of the lower piece of glass is out of phase with the wave reflecting off the bottom surface of the upper piece of glass.
This is discussed on page 552, and shown in Figure 29.23.
