Each colored ring is like a contour line (isoline) on a topographic map.
Work Step by Step
Light reflects from the top and bottom surfaces of the thin film of oil or gas. Depending on the thickness of the film at that location, different colors will reinforce or cancel. Therefore, a color corresponds to a particular thickness of the gasoline film. This is similar to a line on a contour map that joins points of equal elevation/height. This is discussed on pages 553-554, and shown in Figure 29.26.