Abstract
Starting from a description of mode propagation in a homogeneous medium, coupling between different modes at an interface is derived based on a general formulation of the boundary conditions for a medium without spatial dispersion. The results are then applied to a layered medium which yields a simple way to derive the first‐order coupled differential equations for a multi‐mode, inhomogeneous medium in the case of vertical incidence. By this method the results are easily interpreted. As an application the magnetoionic medium is studied.