  We prove, by realistic microscopic calculations within the sp^3s^* Tight
Binding method for GaAs (110) and (100), that the surface optical properties
are not influenced by long-range crystal termination effects, and hence that
they can be consistently studied considering slabs of limited thickness (20 -
30 \AA). The origin of derivative-like and bulk-like lineshapes in Reflection
Anisotropy Spectra is also discussed, analyzing the effects arising from
possible surface-induced reduction, broadening, and shifting of the bulk
spectrum near the surface.
