We compute the linear optical properties of different reconstructions of the clean and hydrogenated Si(100) surface within DFT-LDA, using norm-conserving pseudopotentials. The equilibrium atomic geometries of the surfaces, determined from self-consistent total-energy calculations within the Car-Parrinello scheme, strongly influence reflectance anisotropy spectra, showing differences between the $p(2\ifmmode\times\else\texttimes\fi{}2)$ and $c(4\ifmmode\times\else\texttimes\fi{}2)$ reconstructions. The differential reflectivity spectrum for the $c(4\ifmmode\times\else\texttimes\fi{}2)$ reconstruction shows a positive peak at $\ensuremath{\Elzxh}\ensuremath{\omega}<1 \mathrm{eV}$, in agreement with experimental results.