The three independent components of the piezo-optical tensor P ijkl (ω) have been determined in uniaxially stressed Si using rotating-analyzer ellipsometry. This tensor, with only three complex independent components in the case of Si, links the changes in the real and imaginary parts of the dielectric tensor Δe ij (ω) to an arbitrary stress X=X ki [i.e., Δe ij (ω)=P ijkl (ω)X kl ]. Using the experimental values of P ijkl (ω), several related functions and parameters were derived and compared with previous piezoreflectance, ac-stress-modulated reflectivity, Raman spectroscopy work, and theoretical estimates