We present reflectance-anisotropy spectroscopy (RAS) and surface differential reflectivity (SDR) experiments carried out on well-characterized single-domain nominal silicon (100) surface at room temperature. The results are compared with first-principles calculations which include many-body effects, namely, self-energy and excitonic effects. These strongly modify the line shape of the optical spectra. For a set of the five measurements, the RAS of clean, monohydride, and dyhidride surfaces, and the corresponding SDRs, good agreement between theory and experiment is obtained only when, in the calculation, the electron-hole interaction is accounted for.