Optical properties of Si single crystals with different orientations (1 0 0) and (1 1 1) were investigated using spectrophotometric measurements in a spectral range of 200 nm to 2500 nm. The data of optical absorption revealed an indirect allowed transition with energy gap of 1.1 ± 0.025 eV. An anomalous dispersion in refractive index. The normal dispersion of the refractive index was discussed according to Wemple-DiDomenico single oscillator model. The oscillator energy Eo, dispersion energy Ed, high frequency dielectric constant ∈∞, lattice dielectric constant ∈