  We present first-principles calculations of many-electron effects on the
optical response of graphene, bilayer graphene, and graphite employing the
GW-Bethe Salpeter equation approach. We find that resonant excitons are formed
in these two-dimensional semimetals. The resonant excitons give rise to a
prominent peak in the absorption spectrum near 4.5 eV with a different line
shape and significantly redshifted peak position from those of an absorption
peak arising from interband transitions in an independent quasiparticle
picture. In the infrared regime, our calculated optical absorbance per graphene
layer is approximately a constant, 2.4%, in agreement with recent experiments;
additional low frequency features are found for bilayer graphene because of
band structure effects.
