The paper considers Bose-Einstein condensation (BEC) of light in a cavity with a medium. In the framework of a two-level model we show the effect of gaseous medium on the critical temperature of light condensation in the system. Transition of the system to the state with released light condensate is illustrated in consequent stages. Analytical expressions for a typical spatial extent of the condensed cloud of photons, as well for spectral characteristics of the condensate peak are derived. Energy and heat capacity of photons as functions of temperature are obtained. Finally, we demonstrate that the energy of light can be accumulated in the BEC state.