In the second system, therefore, at each frequency, the walls must absorb and emit energy in such a way as to maintain the black body distribution. Hence absorptivity and emissivity must be equal. The absorptivity α α λ λ of the wall is the ratio of the energy absorbed by the wall to the energy incident on the wall, for a particular wavelength. Thus the absorbed energy is α α λ λ E b λ λ ( λ λ , T ) where E b λ λ ( λ λ , T ) is the intensity of black-body radiation at wavelength λ λ and temperature T . Independent of the condition of thermal equilibrium, the emissivity of the wall is defined as the ratio of emitted energy to the amount that would be radiated if the wall were a perfect black body. The emitted energy is thus ε ε λ λ E b λ λ ( λ λ , T ) where ε ε λ λ is the emissivity at wavelength λ λ . For the maintenance of thermal equilibrium, these two quantities must be equal, or else the distribution of photon energies in the cavity will deviate from that of a black body. This yields Kirchhoff's law :