If you notice the interference patterns of a thin film of oil or gasoline on water you'll note the colors form complete rings how are these rings similar to the lines of equal elevation on a contour?

Answer 1

They are lines of roughly equal film thickness. A given color is maximally reflected when both the primary and secondary reflections are in phase at the wavelength of that color. The phase change of the secondary reflection relative to the primary one must result in a net zero phase relationship. With oil or gasoline on water, the tworeflections themselves produce identical phase changes, so the two-way light path through the film must be an integer number of wavelengths