I have a side question about the diagram on the right. What is the angle phi here, and if it equals phi + pi, then isn't it the same as reflection? I wonder if there is a name for this diagram so that I can find more info about it.
I think it denotes the incident ray and exiting ray in vacuum s.t. they are just ray with reverse directions.
@wcyjames Phi represents the phase of a light since light is a basically a wave and can be denoted by a periodic function with angle phi. So the graph on the right shows the relative path difference (or phase shift) traveled between two rays that pass through different mediums from the same object point is pi.