A black (nonreflective) plane at temperature Tu is parallel to a black plane at temperature Tl. The net energy flux density in vacuum between the two planes is Ju = σB(T4u – T4u), where σB is the Stefan-Boltzmann constant used in (26). A third black plane is inserted between the other two and is allowed to come to a steady state temperature T. Find T in terms of T and T,, and show that the net energy flux density is cut in hail because of the presence of this plane. This is the principle of the heat shield and is widely used to reduce radiant heat transfer.