Inclusion of the conventional classical theory of radiation reaction in the standard classical microscopic model of a material medium induces negligible corrections to the Kramers–Kronig dispersion relations for the complex dielectric constant, and predicts a new ultrahigh-frequency behavior, the same for all media, for any optical property associated with the medium, dependent only on the electron-number density in the medium. One conventional application of dispersion relations to the logarithm of the normal incidence complex reflectivity at a plane interface is shown to be incorrect in principle, regardless of whether radiation reaction is considered. New, correct dispersion relations, to which the conventional ones may be very good approximations in some cases, are given for this quantity. Quantum-field-theoretic considerations also show that the new relations are valid.
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