Low-dimensional silicon structures were grown by atom deposition on CaF2(111) substrates and analyzed by scanning force microscopy as well as by optical spectroscopy. By varying the substrate temperature, T, during growth, structures with different character were generated. At 300 less than or equal to T less than or equal to 700 K, films were formed, above 800 K, a dense overlayer of small clusters was obtained. In contrast, larger separated clusters grew at T > 1000 K. The optical extinction of the samples was measured for lambda = 200-1500 nm, and characteristic spectra were obtained for the three morphologies. The spectra of the films have a broad maximum at lambda = 320 nm and a tail extending throughout the visible. The cluster samples, on the other hand, have a weak extinction in the visible that increases strongly, almost monotonously, with shorter wavelengths. To analyze the influence of the geometry of the Si structures, spectra were simulated using standard electrodynamic formalism. For both the films and the clusters, there is good qualitative agreement between the experimental and calculated spectra.