What is the minimum possible thickness of the gasoline layer directly beneath you?

Question

Looking straight downward into a rain puddle whose surface is covered with a thin film of gasoline, you notice a swirling pattern of colors caused by interference inside the gasoline film. The point directly beneath you is colored a beautiful iridescent green. You happen to remember that the index of refraction of gasoline is 1.39 and that the wavelength of green light is about 538nm.

What is the minimum possible thickness of the gasoline layer directly beneath you?

Answer 1

The 538nm wavelength reflected from the petrol/water interface is interfering constructively with the same wavelength reflected to your eyes from the air/petrol interface, so the thickness of the petrol must be at least half a wavelength, which is 538 x 1.39/2=374 nm.