Why does phase shift depend on whether reflections are external or internal?

[good_phy]

Hi, I learned optics with pedrotti optics book.

It is suppopsed that light is reflected on surface of first layer and

then tranmited wave is also reflected on surface of second layer, So there is Double Layer

incident light get there.


According to book, If two reflections are both external reflection or internal reflection,

Phase shift is not occurred (Assume thickness of layer alone is well to make OPL(Optical Path

Length) of two beam in phase)


But If two reflections are either external or internal, Phase shift of Pi occurred.


I know boundary condition of medium for EM wave create phase shift of Pi when wave is

reflected on the surface of some medium


But this knowledge alone is not able to explain phenomena mentioned above.


Please Solve my question.

 

[Doc Al]

I know boundary condition of medium for EM wave create phase shift of Pi when wave is reflected on the surface of some medium

For light of normal incidence, the phase shift upon reflection depends on the refractive index of the medium: If light reflects from a medium of greater refractive index, there will be a phase shift of pi; if it reflects from a medium of lower refractive index, there is no phase shift.

(I'm not sure if this addresses your question, but it might help.)