Reflection and transmission of electromagnetic radiation is governed by the frequency dependent dielectricpermittivity tensor V(w), in some cases with contributions from the magnetic permeability tensor ' (co). These arefundamental linear response functions, so the corresponding spectroscopy occupies a central position in the study ofsolids. Generally speaking, t3 and U) are the natural point of contact between theory and experiment.
D. R. Tilley,
"Far infrared spectroscopy of manufactured solids", Proc. SPIE 2104, 18th International Conference on Infrared and Millimeter Waves, 210405 (30 August 1993); doi: 10.1117/12.2298437; https://doi.org/10.1117/12.2298437