  We derive the polarization tensor of graphene at nonzero temperature in
(2+1)-dimensional space-time. The obtained tensor coincides with the previously
known one at all Matsubara frequencies, but, in contrast to it, admits analytic
continuation to the real frequency axis satisfying all physical requirements.
Using the obtained representation for the polarization tensor, we develope
quantum field theoretical description for the reflectivity of graphene. The
analytic asymptotic expressions for the reflection coefficients and
reflectivities at low and high frequencies are derived for both independent
polarizations of the electromagnetic field. The dependencies of reflectivities
on the frequency and angle of incidence are investigated. Numerical
computations using the exact expressions for the polarization tensor are
performed and application regions for the analytic asymptotic results are
determined.
