In order to determine the deviations of the Clausius-Mossotti relation due to the changes of the local field close to the surface of cubic crystals, we calculate the optical properties of a semi-infinite cubic lattice of polarizable molecules using two different methods : a perturbative approach and the superposition of bulk modes. We discuss the regions of validity of the perturbative approach and we conclude that there is a surface induced anisotropy in the reflectance of light which could be detected by differential reflectance spectroscopy