Photoelasticity is considered a useful measurement tool for the non-destructive and contactless determination of mechanical stresses or strains in the production of silicon wafers. It describes a change in the indices of refraction of a material when the material is mechanically loaded. As silicon has a diamond lattice structure, the stress-dependent change in the refractive indices varies with the loading direction. In this work, an anisotropic stress-optic law is derived, and the corresponding stress-optical parameters are measured using a Brazilian disc test. The parameters were determined to be (π11-π12) = 14.4 . 10^-7MPa^-1and π44 = 9.4 . 10^-7MPa^-1. The results of this work are compared to previous works found in the literature, and the deviations are discussed.