1. The problem statement, all variables and given/known data I've not been able to do this question for years so I'd really appreciate some help. Light is normaly incident from a medium 1 with impedance Z1 through a layer of medium 2 of thickness L and impedance Z2 into medium 3 of impedance Z3. Obtain an expression for the total reflected intensity of light when the thickness corresponds to: a) λ2/4 b) λ2/2 where λ2 is the wavelength of the light in medium 2. 2. Relevant equations I know that the tangential components of the E and B fields are continuous over the boundaries so at each interface: E1f + E1b = E2f +E2b = E3f H1f + H1b = H2f +H2b = H3f where f and b signify forward and backward going waves Also, E1f/H1f = Z1 And E1b/H1b = -Z1 I know how to do this for the one boundary problem where you get r = (Z2 - Z1)/(Z1+Z2) and t = 2Z2/(Z2+Z1) But I can't apply it to the first boundary as now there's a wave coming the other way being reflected from the second boundary. It all seems intractably complicated but I know this is a fairly standard problem, so I'm finding it really frustrating. 3. The attempt at a solution I don't really even know how to start. The wave in the middle material will bounce back and forwards seemingly infinitely and I don't know how to deal with that. I've just said in the middle there's the sum of a forward going and a backward going wave. I don't really know if the equations above are right for the middle section.