Nonlinear Variation of Refractive Index of Vitreous Silica with Pressure to 7 Kbars

Abstract

The variation of refractive index of vitreous silica with purely hydrostatic pressure to 7 kbars was determined from the shift of the localized interference fringes across the specimen kept in an optical pressure bomb. Allowance for the change in the thickness of the specimen was made with the help of Murnaghan's finite‐strain theory and the recently determined third‐order elastic constants. The refractive index for λ 5893 A increases linearly with pressure with a slope of 9.2 × 10 −4 /kbar up to 4 kbars; thereafter it increases nonlinearly. The increase in the refractive index, however, is linear with respect to the Lagrangian strain in the range investigated, except for a slight tendency for departure from linearity at the highest pressures. No hysteresis effects caused by permanent densification were noted. The results are interpreted in the light of Mueller's theory of photoelasticity of amorphous solids.