Universal Dynamic Conductivity and Quantized Visible Opacity of   Suspended Graphene
  We show that the optical transparency of suspended graphene is defined by the fine structure constant, alpha, the parameter that describes coupling between light and relativistic electrons and is traditionally associated with quantum electrodynamics rather than condensed matter physics. Despite being only one atom thick, graphene is found to absorb a significant (pi times alpha=2.3%) fraction of incident white light, which is a consequence of graphene's unique electronic structure. This value translates into universal dynamic conductivity G =e^2/4h_bar within a few percent accuracy. 
R.R. Nair, P. Blake, A.N. Grigorenko, K.S. Novoselov, T.J. Booth, T.   Stauber, N.M.R. Peres, A.K. Geim