A theory of electric and magnetic birefringence in liquids

Abstract

The double refraction exhibited by liquids when placed in an electrostatic field is ascribed in the theory of Langevin to an orientation of the molecules produced by the field, the orientative couple arising from an assumed electrical anisotropy of the molecule. If an optical anisotropy of the molecule is postulated in addition, the birefringence of the liquid follows as a necessary consequence. Born modified the Langevin theory by including also the orientative effect of the field on the molecule due to the permanent electric moment, if any, possessed by it. The optical anisotropy and electrical polarity of the molecule postulated in these theories can be independently determined from observations of light-scattering and dielectric constant in the vapours of the substances. In two recent papers we have attempted to discuss how far the available data for the Kerr effect can be reconciled with the theories of Langevin and Born. The main result emerging is that these theories fail to give the magnitude of the Kerr constant in liquids correctly in terms of the constants of the molecule as determined in the gaseous condition. This failure is illustrated in Table I for a number of liquids having non-polar molecules for which the optical anisotropy is known from observations on light-scattering in the vapour. It appears hardly likely that the failure of the Langevin theory indicated by the figures in Table I can be ascribed to a real change in the optical anisotropy of the molecule when it passes from the condition of vapour to that of liquid. It seems rather that the explanation must lie in the inadequacy of the theory itself. We accordingly propose in this paper to put forward a theory of electric and magnetic double refraction, in which the fundamental premises of Langevin and Born are revised.