In a Fabry-perot interferometer, light is reflected back and forth between 2 highly reflecting parallel mirrors, with a nonconducting medium inside. The waves of magnetic and electric field are 90o out of phase, unlike the case of a wave in free space where they in phase. Develop an argument using Poynting's theorem for why this should be so.
The Attempt at a Solution
I've thought of using E to calculate H and then say that they're out of phase, but then this method isn't based on Poynting's theorem.
Then I've thought of using the eq.:
E = E0exp((-kapper)z+i(kz-wt))
B = B0exp((-kapper)z+i(kz-wt+theta))
to calculate the Poynting vector, S = 1/mu * (E x B), then somehow show that theta is 90o, but then I don't really know how to do it.