Photo-elastic behaviour of ammonium alum crystals

Abstract

To test an earlier prediction by Bhagavantam, based on a group theoretical method, that the T-23 and T_k-2/m 3̅ classes of the cubic system need four independent constants, instead of three, for describing their photo-elastic properties, crystals of ammonium alum belonging to the T_k - 2/m 3̅ class were studied completely. It was found that, the crystal needs four independent constants and all the four stress-optical constants of alum have been determined. For sodium D light, they are (q₁₁-q₁₂)=-5.93 × 10^-13, (q₁₁-q₁₃)= -5.25 × 10¹³, q₄₄=-1.15 × 10¹³, q₁₁= 5.5 × 10^-13, q₁₂=11.6 × 10¹², q₁₃= 10.9 × 10^-13 cm.² dyno^-1. The values of the strain-optical constants are (p₁₁-p₁₂)= -0.0854, (p₁₁-p₁₃)=-0.0756, p₄₄=-0.0092. p₁₁=0.38, p₁₂=0.40, p₁₃=0.45.