Blue light of wavelength 480 nm is most strongly reflected off a thin film of oil on a glass slide when viewed near normal incidence Assuming R.I. of oil is 1.2 and that of glass is 1.6 find  minimum thickness of the oil film (other than zero).

Oil is Optically denser than air, also glass is optically denser than oil. So the incident rays at top and bottom surfaces of oil film, both suffer an additional phase difference of 180^o upon reflection.

Therefore, for the formation of bright fringe

For near normal incidence,