Physical background of photoelasticity phenomenon

This phenomenon is the basis for the photoelasticity method of studying inner stresses in transparent materials. 

An ordered packing of macromolecules may also cause an optical anisotropy and birefringence, which are characteristic, for instance, of polymer spherulites. Because of the radial anisotropy of a spheruHte and the convergence of beams in the spherical structure, the interference picture represents the so-caUed Maltese cross, the center of which is located in the center of spherulite. No calculations are performed using such a picture but the photoelasticity method is very efficient in revealing qualitatively the presence of any spherulites, or a mesomorphic or ordered sate of polymeric chains. 

In a polarizing optical microscope with crossed polarizer and analyzer, a transparent spherical isotropic sample looks like a bright object on the dark 

---