A film of gasoline (n=1.40) floats on water (n=1.33). Normally incident blue light of a wavelength 480 nm, appears bright blue due to constructive interference. Determine the minimum non-zero thickness of the film

Answer 1

waves reflected from the gasoline/water interface
... are in phase with the incident light

this means the minimum film thickness is half a wavelength

Answer 2

so which formula should i use?