Optical constants of graphene layers in the visible range

We show that the optical constants of graphene in the visible range can be estimated by means of a very simple procedure involving their consistence with universal optical conductivity and experimentally measured optical spectra, within the framework of Fresnel coefficients calculation. The obtained complex refractive index allows for accurate prediction of the optical behavior of graphene in the visible range, from the two-dimensional limit (single atomically thick graphene layer) to the bulk limit (graphite). Therefore, it may result very useful for quantitative optical analysis of graphene layers and graphitic structures in general.