AbstractThe birefringence exhibited by high polymers can be explained by considering light as an electromagnetic wave propagating through an anisotropic dielectric. The observed dependence of the birefringence on the deformation stale is described by expressing the dielectric tensor as a functional of the strain history. For small strains, the functional can be approximated by an integral so that the birefringence and strain are related by a law of the Boltzmann superposition integral form.