Effect of many body forces on the photoelastic constants of the alkali halides

A simple phenomenological lattice theory based on the Lundqvist potential of ionic cohesion (many body theory of elastic dielectric), to explain photoelastic behaviour of the solids with rock-salt structure, is presented and shown to apply within reasonable errors to the known experimental constants. The predicted values of the photoelastic constants of the alkali halides are very close to the experimental values and are better than obtained by all earlier workers.