1. The problem statement, all variables and given/known data 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. 2. Relevant equations 3. 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.