Organic salt kappa-(BETS)
is a unique compound in which local moment
antiferromagnetism of Fe
ions (below Neel temperature T
of 2.5 K)
bulk superconductivity (below the superconducting transition temperature of T C =1.1 K). After a general introduction to the optical properties of conventional superconductors I will discuss the results of our experimental investigation of the optical properties of kappa-(BETS) 2 FeBr 4 in which we probe the temperature regime of the coexistence as well as the normal state.