Detailed studies of the often-observed resonance feature near 3.4 eV in the reflectance difference spectrum ofSi/SiO2interfaces and Si surfaces show that the resonance is due to the intrinsic local-field effect, and that its energy position coincides with theE1energy of bulk Si. Using the energy position of the resonance of the pseudomorphically grownSi/CaF2interfaces as a reference point, the strain-induced resonance energy shift of the Si interface layer at severalSi/SiO2interfaces are obtained and the strain in these layers is determined. The results show that the interface layers are highly strained with an equivalent hydrostatic pressure of 0.79 GPa, but still maintain a high degree of order. ©1997 American Institute of Physics.