Photoelastic constants of diamond
All the photoelastic constants of diamond have been determined for the first time. The three stress-optic coefficients areq11=4·2×10−11,q12=−3·6×10−11,q44=2·6×10−11, from which the elasto-optic coefficients have been deduced to bep11=0·125,p12=−0·325,p44=0·11. It is found thatq11−q12 andq44 are both positive and also that the refractive index of the crystal should decrease when subjected to a hydrostatic pressure, both of which are unique for diamond among the cubic crystals studied so far. In the course of the investigation, a new technique has been developed for determining the absolute path retardation, based on the production of localised interference fringes by the light coming from the two surfaces of the crystal.