An exact knowledge of the mechanical and optical properties of crystals allows not only for theoretical advances, but it is also a useful tool to asses crystal quality in the technological processes of growth and production of advanced crystals like, eg, scintillators. In this paper, we study the elasto‐optic behavior of tetragonal crystals to evaluate the photoelastic constants, associated to various states of stress, in terms of the components of the piezo‐optic tensor. Moreover, we arrive at a generalization, for tetragonal crystals, of the Brewster law for optically isotropic materials.