General boundary conditions for the wave equation around non-homogenous scatterers

To solve the wave equation inside a region that contains an inhomogenous dielectric material of arbitrary shape under the influence of an incoming wave, we establish a generalized boundary condition. The solutions inside a finite region resulting form a given incoming wave from the outside, are determined by a linear relation between the normal gradient and the function values on the boundary. This boundary condition is non-local and we show how it can be used in conjunction with the variational principle applied to an open system.