Dispersion and Refractive Index of Nitrogen Measured as Functions of Pressure by Displacement Interferometry

Abstract

The Lorentz-Lorenz relation has never been satisfactorily checked by experiment for pressures above atmospheric, because of a lack of trustworthy data on the refractive indices of pure gases as functions of pressure. Such data would also be particularly valuable in the study of polar molecules on account of Maxwell's law connecting the refractive index with the dielectric constant. Due, however, to the difference between optical and electrical frequencies the refractive index must be reduced to zero frequency before Maxwell's law can be applied. This necessitates a precise knowledge of dispersion which is not available for many pure gases. In the present paper a method of measuring simultaneously the dispersion and refractive index of a gas is discussed. The method is justified on theoretical and experimental grounds and applied in a practical way to the measurement of dispersion and refractive index of nitrogen over a range of pressures and at two different temperatures with a high degree of precision.