A molecular theory of linear birefringence induced by crossed electric and magnetic fields

A symmetry method is used to confirm the existence of a previously reported but as yet unobserved linear birefringence, which is induced in a fluid by the simultaneous application of an electric field E and a magnetic field B perpendicular to each other and to the light beam. For weak fields the effect is proportional to EB. Also presented is a quantitative theory which relates the birefringence in an ideal gas to various multipole polarizability tensors of its molecules. The expression obtained is identical to that for another but essentially different linear birefringence, which is induced in a fluid by parallel electric and magnetic fields acting at right angles to the light path. For reasons relating to the fast and slow axes of these two birefringences, that induced by the crossed fields offers a more convenient means to the available molecular information. The existence in ferroelectric crystals of birefringence induced by and linear in an applied magnetic field is inferred from the molecular result and the classes of uniaxial crystal which may exhibit this effect are listed.