Silicon dioxide layers on crystalline silicon substrates were thermally grown under different oxidation conditions. The ellipsometric functions (psi and Delta) of the silicon dioxide layers were measured by using variable-angle spectroscopic ellipsometry (VASE). The effective refractive index of the silicon dioxide layer decreases from 1.544 to 1.458 as the layer thickness increases from 12 to 150 nm. The critical behavior of the s- and the p-wave antireflection condition has been observed from the experimental VASE data in accordance with the predicted sensitivity graphs based on an interface layer between the crystalline silicon substrate and the oxide.