  Optical reflectivity and transmission measurements over photon energies
between 0.2 and 1.2 eV were performed on single-crystal graphene samples on a
transparent SiO2 substrate. For photon energies above 0.5 eV, graphene yielded
a spectrally flat optical absorbance of (2.3 +/- 0.2)%. This result is in
agreement with a constant absorbance of pi*alpha, or a sheet conductivity of
pi*e^2/2h, predicted within a model of non-interacting massless Dirac Fermions.
This simple result breaks down at lower photon energies, where both spectral
and sample-to-sample variations were observed. This "non-universal" behavior is
explained by including the effects of doping and finite temperature, as well as
contributions from intraband transitions.
