We report the optical properties of amorphous C:Cu films produced by co-sputtering of graphite and copper targets in an argon plasma. Ellipsometric measurements at 632.8 nm were performed to evaluate dielectric permittivity vs copper content. We have observed a pronounced peculiarity of imaginary and real parts of the dielectric function at a copper content of about 14%. A correlation between the concentration dependence of the DC conductivity and that of the imaginary part of permittivity is clearly demonstrated. This correlation points to the existence of a less-conductive phase with the stoichiometric formula CuC6.