Since radiation within a cavity propagates isotropically with velocity c, the flux of energy impinging on unit area of the wall (or passing in one direction through an imaginary unit surface within the cavity) is given by the ``Stefan--Boltzmann Law'':
Energy flux per unit area=41c(U/V)=41cbT4≡σBT4
The factor of c/4 arises as 21(c/2); the factor of 21 selecting only the radiation crossing the imaginary area from ``right'' to ``left'' (or vice versa), and the factor of c/2 representing the average component of the velocity normal to the area element. The constant σB (=cb/4) is known as the ``Stefan--Boltzmann constant.'' As an exercise in elementary kinetic theory, derive the Stefan--Boltzmann law (explicitly demonstrating the averages described).