Über Gruppengeschwindigkeit, Energiestromdichte und Energiedichte in der Röntgen- bzw. Lichtoptik der Kristalle

Abstract

Studying the propagation of X-rays (or visible light) in perfect crystals, the wellknown expression for the group velocity vector can be used and interpreted in terms of the dispersion (or index) surface. The group velocity vector, being normal to the dispersion surface in the point representing the appropriate wavefield, is shown to be proportional to the average of the Poynting vector observable macroscopically. The factor occuring in this relation is the averaged energy density of the electroagnetic field, which differs from the usual expression by a term proportional ∂ɛ/∂ν as a consequence of the frequency dependence assumed for the dielectric tensor ɛ.