Hi, I think I'm missing something with how a parallel plate waveguide works. In the picture I've shown below, there intensity of the E-field change depending on how far across the waveguide it is. While I know that this has to happen in order for anything to propagate, I don't quite understand why it is happening. If the top plate is connected to an oscillating voltage source, and the bottom plate is grounded, shouldn't the bottom plate always be at 0 and shouldn't the E-field of the whole top plate float up and down with the voltage source? I would've thought that at an instant in time, the E-filed would have the same magnitude everywhere in dielectric (as the bottom plate is constant 0V, and the whole top plate would float up and down together). Why, at a particular instant in time, does the E-field magnitude change through the dielectric? Thanks.