The History of the Mixed Poynting Vector
This discussion of the importance of the mixed Poynting vector is given by way of a remark on the paper of H.-D. Geiler, K. Hehl, and D. Stock in 1983. It can be pointed out that the onset of this discussion can be traced back to the mid 19th century and to Stokes, v. Helmholtz, Lord Rayleigh etc. The mixed Poynting vector appears in all cases of reflection of electromagnetic waves at a boundary between two arbitrary media with different complex refractive indices. The energy flux density in the first medium supporting the incoming radiation is changed by the superposition of the incoming and the reflected wave. In the case where the first medium is absorptive, the interference effects lead to a specific energy saving. This changed energy flux is especially important in the case of laser application because the energy quantities are sufficiently great to be detected and/or to influence the technical or experimental applicability of laser power. In a short recapitulation of the main problems of understanding of the physical situation it is pointed out that this basic phenomenon has already been clarified in corresponding publications from 1950.