English: Tension lines in plastic protractor seen under cross polarized light. When a ray of plane polarised light is passed through a photoelastic material, it gets resolved along the two principal stress directions and each of these components experiences different refractive indices. The difference in the refractive indices leads to a relative phase retardation between the two component waves. The birefringence of the plastic layer shows colored fringes. Also called Photoelasticimetry or photoelasticity. Not seen does not mean No difference.