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The porous silicon (PS) layers with different porosities were prepared by electrochemical anodization method to obtain the desired refractive index in a given range. The optical response of the PS layer has been performed by the reflectance measurements. The obtained data were coupled to Kramers-Kronig analysis and allowed determination of complex refractive index in spectral range extending from 400 to 850 nm. In addition, the dispersion of refractive index of PS was determined from optical reflection measurements experimentally utilizing the relationship between the optical path in the plane-parallel film and the position of extrema. These results were compared and the agreement of them was discussed. The Bruggeman’s effective medium approximation was also used to estimate the porosity of each PS sample from calculated optical characteristics. FESEM analyses revealed vertically aligned silicon nanopillars with diameters on the order of 50 nm perpendicular to the Si substrate surface. A gradient-porosity PS layer was fabricated by modulating applied current density during anodization. Indeed, the elaborated structure is consisted of porous layers with modulated refractive indices. For this structure, normal incident and reflecting light through the layers provide an effective refractive index.