We study the structure and optical properties of arrays of silicon nanowires (SiNWs) with mean diameter of 100 nm and length of about 1-25 µm formed on crystalline silicon (c-Si) substrates by using metal-assisted chemical etching in hydrofluoric acid solutions. In the middle infra-red spectral region the reflectance and transmittance of the formed SiNW arrays can be described in the framework of an effective medium with the effective refractive index of about 1.3 (porosity ~75%), while a strong light scattering for wavelength of 0.3÷1 µm results in a decrease of the total reflectance to 1-5 %, which cannot be described in the effective medium approximation. The Raman scattering intensity under excitation at ~1 µm increases strongly for the sample with SiNWs in comparison with that for c-Si substrate. This effect is related to an increase of the light-matter interaction time due to strong scattering of the excitation light in the SiNW array. The prepared SiNWs are discussed as a kind of “Black Silicon”, which can be formed in a large scale and it can be used for photonic applications as well as in molecular sensing.