A tank of gasoline having n = 1.40 is open to the air having n = 1.00. A thick film of liquid floats on the gasoline and consists of a refractive index which is between 1.00 and 1.40. Light that consists of a wavelength of 634 nm (in vacuum) shines perpendicularly down via the air onto this film, and this light looks bright due to constructive interference. The thickness of the film is around 254 nm and is the minimum nonzero thickness for which constructive interference can take place. Determine the refractive index of the film?