Variational theory of the reflection of light by interfaces

Schwinger’s variational theory of scattering is adapted to the calculation of reflection amplitudes. For an arbitrary transition between two media at a planar interface, we derive variational expressions for the s and p reflection amplitudes that are correct at grazing incidence and correct to second order in the ratio of interface thickness to wavelength. The interface or the substrate (or both) may be absorbing. The Hulthen–Kohn variational theory of is also adapted to reflection. The results are simpler (for the same trial wave function) but not so good as scattering those obtained from the Schwinger method.