Answer
Interference of light waves accounts for the iridescence of a film of gasoline.
Work Step by Step
When gasoline is floating on water, light waves bouncing off the top gasoline surface and off the underlying water surface will travel different distances, and interfere with each other. This results in a shimmering spectrum of light, as some colors are in phase and interfere constructively, while others are out of phase and interfere destructively.
On a dry street, there is no water surface for light to bounce from.
This is discussed on pages 553-554, and shown in Figure 29.26.
