Nonlinear Variation of the Refractive Indices of α-Quartz with Pressure*

The variations of the ordinary and extraordinary refractive indices of α-quartz with purely hydrostatic pressure to 7 kbar have been determined from the shift of the localized interference fringes across the crystal under fluid pressure. Allowance for the change of thickness of the specimen was made with the help of the nonlinear theory of elasticity. The ordinary and the extraordinary refractive indices for λ5893 Å increase linearly with pressure with slopes of 1.03×10−3/kbar and 1.075×10−3/kbar, respectively, up to about 4 kbar; thereafter they increase nonlinearly with pressure. The increase of the refractive indices however, is found to be linear with respect to the Lagrangian strain in the entire range investigated. The results are compared with similar data on vitreous silica and are also interpreted in the light of Mueller’s theory of photoelasticity.