A tank of gasoline of n = 1.40 is open to the air of n = 1.00. A thick film of liquid floats on the gasoline and consists of a refractive index which is among 1.00 and 1.40. Light that consists of a wavelength of 652 nm (in vacuum) shines perpendicularly down through the air onto this film and this light looks bright due to the constructive interference. The thickness of film is around 260 nm and is the minimum non-zero thickness for which constructive interference can take place. Determine the refractive index of the film?