A film of index of refraction of 1.4 and thickness of 1.2 x 10-5m is on a

Question:

A film of index of refraction of 1.4 and thickness of 1.2 x 10-5m is on a lens with an index of refraction of 1.6. Light of wavelength 600 nm is incident normally from air to the film. Consider only reflections from the top and bottom surfaces of the film.
(a) How many reflected waves will experience the 180o phase shift?
(b) What is the path length difference between the two reflected waves?
(c) Will the reflected waves interfere constructively or destructively?