The reflection coefficient of a symmetric parallel-plate waveguide operating in the TEM mode and illuminating a lossless dielectric layer is analyzed by wedge diffraction and geometrical optics techniques. The interactions between the waveguide and the dielectric layer are treated in terms of iterative bounce waves. The reflection from the dielectric layer and the scattering from the waveguide for cylindrical wave incidence are both represented by resultant component cylindrical waves. Thus each interacting bounce wave between the guide and the dielectric layer becomes the superposition of a finite number of cylindrical wave components. The reflection coefficient of the guide is then obtained by adding the various bounce wave contributions to the guide's free-space reflection coefficient. For the two geometries considered, the thin-walled guide and the ground-plane guide, calculated results are in excellent agreement with those measured.