New data on the dependence of the function of the refractive index of soot particles on their average size in the visible and near-IR spectral regions are presented. The data are obtained by laser-induced incandescence. The sunlight absorption by soot aerosols is currently calculated with a value of the refractive index function of about 0.2 at a wavelength of 550 nm regardless of the origin of soot particles and their variability. The data demonstrate that optical properties of soot depend on the size of particles and conditions of their formation, which, in turn, is related to the degree of their graphitization. It is shown that taking into account the particle size distribution in soot aerosols can lead to an increase in solar radiation absorption by them approximately by two times as compared to the commonly accepted values.