Interference and the Wave Nature of Light

A tank of gasoline (n = 1.40) is open to the air (n = 1.00). A thick film of liquid floats on the gasoline and has a refractive index that is between 1.00 and 1.40. Light that has a wavelength of 660 nm (in vacuum) shines perpenducularly down through the air onlto this film, and this light looks bri…