Photoelastic Constants of Vitreous Silica and Its Elastic Coefficient of Refractive Index

The photoelastic constants and their dispersion for visible light were determined for two samples of vitreous silica, Corning and Herasil No. 1. The results for the mercury green line, 546 mμ, are quoted here. The photoelastic constants were 4.22×10−13 (d/cm2)−1 (ordinary ray) and 3.56×10−13 (d/cm2)−1 (extraordinary ray). From the ratio of Poisson's ratio to Young's modulus 0.216×10−12 (d/cm2)−1 and Young's modulus 0.76×1012 (d/cm2), which were also determined, there were calculated the pressure coefficient of refractive index 0.909×10−12 (d/cm2)−1 and the elasticity volume coefficient of refractive index V dN/dV=0.34. Comparison of these results with those reported for other glasses indicates that the oxygen ions of vitreous silica are more deformable then the oxygen ions of other siliceous glasses. In vitreous silica about ⅓ of the volume change accompanying an elastic dilatation arises from the dilatation of the oxygen ions. A comparison is also made with the results reported for corresponding thermally induced effects. It indicates that (a) during a thermal dilatation of a siliceous glass, the oxygen ion undergoes a dilatation which in the case of most siliceous glasses causes an equivalent dilatation of the body, but which in the case of vitreous occurs internally instead, and (b) in crystal quartz no appreciable dilatation of the oxygen ion occurs during a thermal dilatation. The thermal dilatation of the oxygen ion in the glasses would thus seem related to its strained bond configuration.