11.1 Wave propagation in changing media
For much of our analysis so far, we have been concerned with the propagation of waves through continuous, uniform media. We have then dealt with diffraction at obstacles by adopting the Huygens approach, and we have seen that this can also describe the reflection and refraction that occur at interfaces, for which it both provides an excellent conceptual description of the processes and allows us to calculate the directions of the reflected and refracted waves.
Pain  pp. 117–119 and 163–165French  pp. 253–259Main  pp. 299–307
With appropriate mathematical dexterity, ...