In search of next-generation solar cells, silicon nanowire arrays have attracted great attention since they are costeffective and may absorb more light compared to thin-film silicon solar cells. Theoretical studies using finite-different time-domain and transfer matrix methods have been performed to investigate the optical properties of silicon nanowire (SiNW) arrays, however, they are computationally intensive and require periodic condition, which may not be satisfied with most fabricated samples. In the present study, an effective medium analysis is performed to study the optical properties of vertically-aligned SiNWs on Si substrate in the wavelength range from 310 nm to 1100 nm, which is of the most important interest for solar cells. The effective dielectric function of the SiNW layer is obtained from the Bruggeman approximation. Thin-film optics formulae are employed to calculate the reflectance, transmittance and absorptance of the SiNWs on silicon substrate at normal incidence. The effect of geometric parameters such as filling ratio and wire length will be investigated to understand the light absorption and to facilitate the optimal design of highperformance SiNW solar cells.