Photo-elastic behaviour of ammonium alum crystals

To test an earlier prediction by Bhagavantam, based on a group theoretical method, that the T-23 and T_h-2/m  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_h-2/m  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 × 10^-13, q₄₄ = -1.15 × 10^-13, q₁₁ = 5.5 × 10^-13, q₁₂ = 11.6 × 10^-13, q₁₃ = 10.9 × 10^-13 cm.² dyne^-1. The values of the strain-optical constants are (p₁₁ - p₁₂) = -0.0854, (p₁₁ - p₁₃) = -0.0756, p₄₄= -0.0092, p₁₁ = 0.38, p₁₂ = 0.46, p₁₃= 0.45.