The surface and interface effects on the dielectric polarization and refractive indices of BaTiO₃ single crystals and BaTiO₃ ultrathin films on SrTiO₃ single crystal substrates are investigated theoretically by using a microscopic model based on the orbital approximation in correlation with the dipole-dipole interaction. The spontaneous polarization of BaTiO₃ single crystals is drastically reduced near the c-surface. For BaTiO₃/SrTiO₃ films, the spontaneous polarization is reduced in the film as its thickness decreases. However, an electronic polarization appears within the SrTiO₃ substrate in the neighborhood of the interface. This polarization, which vanishes far away from the interface into the SrTiO₃ bulk, is induced by the polarization of the BaTiO₃ film. Furthermore, we find the refractive index either for BaTiO₃ single crystals or for BaTiO₃ films and SrTiO ₃ substrates to be strongly reduced for light polarized perpendicular to the surface.