Photoelasticity of Crystals. Introduction
The subject of photoelasticity deals with the artificial birefringence developed in a solid under the application of a mechanical stress. This phenomenon was first discovered in jellies by Sir David Brewster, who in 1815 presented a paper before the Royal Society, London, announcing the effect. He reported in 1816 the same effect in amorphous solids such as glass and in cubic crystals such as fluorspar and diamond. By 1818 he had also studied the stress-optical behavior of some uniaxial and biaxial crystals. In his earlier investigations, he placed a strip of glass between two crossed Nicols. When the strip of glass was stretched, the field of view brightened up, thereby showing that artificial birefringence was induced in the glass strip. He found that glasses and cubic crystals which are originally optically isotropic acquire the property of double refraction under a mechanical stress. Furthermore, he found that ordinary glass behaves as a negative uniaxial crystal upon the application of compressional stress, and as a positive uniaxial crystal under tension. In the case of solids which are initially birefringent, the initial birefringence is altered by the stress. Though his investigations were of a qualitative nature, he gave a correct interpretation of the stress-optical phenomenon.
KeywordsUniaxial Crystal Biaxial Crystal Rochelle Salt Potassium Alum Photoelastic Constant
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