Stress Optical Coefficients of Different Specimens of Vitreous Silica

Abstract

IN a recent communication¹ the results obtained from a study of the stress optical coefficient of a sample of vitreous silica and its variation with wave-length were described. This sample was obtained from the Thermal Syndicate, Ltd., England, a few years ago. In order to find out whether there was any variation of stress optical coefficient from specimen to specimen, two specimens of vitreous silica one of ‘OH’ quality and the other of ‘OG’ quality were recently obtained from the same firm. JEach specimen was in the form of a rectangular plate (20 mm. × 20 mm. × 5 mm.) with polished faces. Using the same experimental technique as before¹, accurate determinations have been made of the stress-optical coefficient, C, of these two samples over the wave-length range 2300 A.–5400 A. Measurements were made for not less than 25 different wave-lengths well distributed throughout the range investigated. The variation of C with wave-length for the two new specimens is represented graphically in Fig. 1. The errors in the value of C for the various wave-lengths for the ‘OH’ and ‘OG’ quality specimens are less than 0.5 per cent. The values obtained for the earlier specimen are shown as curve P in Fig. 1. It is seen that for any wave-length the stress optical coefficient C for the ‘OH’ quality specimen is less than that for the ‘OG’ quality, the cQfference being of the order 2 per cent. It is found that dispersion for all these specimens remains practically the same.