The physical origin of sharp resonances in reflectance difference spectroscopy (RDS) data at the critical points of the dielectric function of bulk Si, previously assigned to surface-bulk transitions, to photon localization or to optical transitions from bound dimer states to excited dimer states is investigated. We show that uniaxial in plane stress of bulk Si induces sharp resonances at exactly these critical points of Si via the piezo-optic effect. In the recent literature, it was shown that surface reconstruction as well as dimerization exerts anisotropic stress, e.g. along the dimer direction, and the resulting strain is extending into the bulk. In our contribution, we simulate this surface strain by externally stressing different Si faces and compare ex situ measured RDS data of Si(0 0 1), Si(1 1 1) and Si(1 1 0) surfaces with RDS data measured in situ and density functional theory calculations.