Topics
Harmonic waves, Electrical properties and parameters, Electromagnetic radiation, Fundamental constants, Visible spectra, Microwave frequencies, Optical properties, Educational assessment, Public policy and governance, Scholarly publishing

Maxwell's theory of electromagnetic waves in palpable media yielded the relation n2=εr between the refractive index n of a negligibly absorbing medium and its relative permittivity εr=ε/ε0 (the permeability μ that of free space).1 Although he recognized that n and εr must be determined at the same frequency, few measurements were at hand in the 1870s. He did, however, find the published value 1.975 for the static relative permittivity of solid paraffin, which is cloudy and hence no more possesses a refractive index in the visible than ordinary clouds do. But he was able to obtain the refractive index (⁠≈1.43⁠) of liquid paraffin at three Fraunhofer lines, from which he extrapolated n to zero frequency, possibly the largest extrapolation in scientific history. His extrapolated value2 was 1.422 whereas εr was 1.405....

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