The optical properties of Si structures, such as hydrogen-terminated Si quantum wires, Si quantum wires embedded in Ge, and Ge quantum wires embedded in Si, are studied in a unified and consistent way, using an accurate and efficient microscopic electronic structure method. We investigate the dielectric function in a broad energy range. For the hydrogen-terminated wires we observe a splitting of the main peak and near the fundamental bandgap some narrow and strong peaks. For the embedded wires the changes are less dramatic, the main characteristic being a considerable transfer of oscillator strength to lower energy.