English: A cd case placed between two crossed polarizers appears intense and beautiful colors. According to the theory of photoelasticity, the allocation of the colors gives information about the existence and the allocation of stresses in the material of the ruler. These stresses may be a result of external forces or may be permanent stresses because of the procedure the case was manufactured (as it happens in the case of this picture).

English: Photoelasticity and colors on a cd case