A film on a lens with an index of refraction of 1.5 is 1.0 x 10-7 m

Question:

A film on a lens with an index of refraction of 1.5 is 1.0 x 10-7 m thick and is illuminated with white light. The index of refraction of the film is 1.4.
(a) The number of waves that experience the phase shift is (1) zero, (2) one, (3) two. Explain.
(b) For what wavelength of visible light will the lens be nonreflecting?