The pressure of radiation in a refractive medium has been a matter of theoretical controversy for many years, though relatively few experiments have been performed. We have measured the photon drag effect in germanium and silicon in the far-infrared, up to a wavelength of 1.2 mm. At sufficiently long wavelengths the effect is independent of the semiconductor band structure and depends on the radiation pressure in the dielectric. We find that the expression originally deduced by Minkowski correctly describes our results.

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