Solve the all parts of given problem:

Problem: Light with wavelength 648 nm in air is incident perpendicularly from air on a film 8.76 µm thick and with refractive index 1.35. Part of the light is reflected from the first surface of the film, and part enters the film and is reflected back at the second surface, where the film is again in contact with air.

Part I: What is the phase difference between these two parts of the light as they leave the film?

Rework your solution from the starting and check each step carefully. Thanks.