Photoelastic behaviour of ionic crystals

Abstract

A theory of photoelasticity in ionic crystals is proposed wherein the overlap of the electronic atmospheres of adjacent ions and the effect of strain on the ionic polarisabilities are considered the dominant factors. The sign of the stress optical constant predicted by this theory is found to be in agreement with the experimental result. The change in the sign of the stress optical constant at lower wavelengths exhibited by some of the alkali halides is also explained. The ‘pure’ temperature effect observed in the thermo-optic behaviour of crystals turns out to be a natural consequence of the proposed theory.