Dispersion of the Stress Optical Coefficient of the Alkali Halides

Abstract

IN recent years the dispersion of the stress optical coefficients of diamond¹, the alkali halides² and magnesium oxide³ has been determined in the visible range of wave-lengths. The dispersion in fused quartz^4,5 has also been investigated from 5700 to 2500 A. The observed dispersion is of the order of a few per cent for all these materials. In the present investigation the dispersion of the stress optical coefficient C = n ³(q ₁₁ – q ₁₂)/2 has been determined from the visible range to 2400 A. The method of measurement combines the differential feature of Bansigir and Iyengar's² method with the photographic method of Jog⁶. Using fused quartz (presented by the Thermal Syndicate to Prof. R. S. Krishnan) as the standard, the stress optical coefficient of the alkali halides is obtained by determining the position of the dark bands in the spectrum of light polarized at 45° to the direction of stress and passed through the fused quartz specimen loaded suitably, the stressed alkali halide crystal and an analyser in the crossed position. The position of the dark bands is determined by the equation: where C is the stress optical coefficient of the material for wave-length λ; W is the load applied on it; t is its thickness and n is the order of the band. C ₁, W ₁ and t ₁ refer to the standard and C, W and t to the alkali halide used.