A film of oil that has an index of refraction equalto 1.45 floats on water (n = 1.33). When illuminated by white light at normal incidence, light of wavelengths 629 nm and 449 nm is predominant in the reflected light. Determine the thickness of the oil film.

Question

Wouldn't the thickness of the oil film be such that the thickness*2 be a whole multple of wavelengths?

http://physics.bu.edu/py106/notes/Thinfilm.html