How to know phase difference will be of a light wave entering a thin film?

For instance, I know if a light wave traveling air (n = 1) enters a thin film of water (n = 1.33), there will be a phase difference.

My text book sol'n manual states that there will be a 180 degree phase difference for a wave of light that reflects of the top of the air-water boundary, but NO phase difference for a wave that refracts into the thin layer of water and reflects off the bottom of the thin layer.

But the book fails to explain why and how we are supposed to know when a phase difference occurs at the two boundaries, and WHAT the phase difference will be at the two boundaries, and what the net phase difference is. Can anyone help?

 

Never mind I found the answer. If anyone in the future comes looking for the answer to this:

If light goes from a ---> b with n(a) > n(b), the reflected wave has no phase shift

If light goes from a ---> b with n(b) > n(a), the reflected wave has half cycle (180 deg) phase shift

n above is index of refraction. b is the inside of the thin film.

Entering the thin film, most probably nb > na, so 180 deg phase shift.

So at the back of the film (exiting it), the light will probably be going from higher n to lower n, so there's no phase shift