  We investigate the reflectance of a dielectric plate coated with a graphene
sheet which possesses the nonzero energy gap and chemical potential at any
temperature. The general formalism for the reflectance using the polarization
tensor is presented in the framework of Dirac model. It allows calculation of
the reflectivity properties for any material plate coated with real graphene
sheet on the basis offirst principles of quantum electrodynamics. Numerical
computations of the reflectance are performed for the graphene-coated SiO$_2$
plate at room, liquid-nitrogen, and liquid-helium temperatures. We demonstrate
that there is a nontrivial interplay between the chemical potential, energy
gap, frequency, and temperature in their joint action on the reflectance of a
graphene-coated plate. Specifically, it is shown that at the fixed frequency of
an incident light an increase of the chemical potential and the energy gap
affect the reflectance in opposite directions by increasing and decreasing it,
respectively. According to our results, the reflectance approaches unity at
sufficiently low frequencies and drops to that of an uncoated plate at high
frequencies for any values of the chemical potential and energy gap. The impact
of temperature on the reflectance is found to be more pronounced for graphene
coatings with smaller chemical potentials. The obtained results could be
applied for optimization of optical detectors and other devices exploiting
graphene coatings.

Mesoscale and Nanoscale Physics;Quantum Physics