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 occured (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 occured. 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.