The variation of the refractive index of sphalerite with pressure to 7 kbar has been determined by an interferometric method. The refractive index decreases with increasing pressure, as was predicted earlier. Above 3 kbar the change in refractive index is found to be nonlinear with respect to both stress and strain, thus demonstrating that Pockels's linear elasto- and piezo-optic equations have to be extended.

• Received 27 May 1966

DOI:https://doi.org/10.1103/PhysRev.150.766