Rays versus modes: pictorial display of energy flow in an open-ended waveguide

The problem of a plane wave impinging on a semi-infinite parallel-plate waveguide is investigated. It is demonstrated that, for waveguide separation large compared to the wavelength, the fields inside the waveguide obey a simple ray optics description. The beam behavior persists for more than into a waveguide. For a small waveguide, the beam picture begins to blur approximately into the guide.