Two infinite plane parallel black surfaces exchange thermal radiation. A black radiation shield is inserted midway between them. Find the ratio of heat-transfer rates with and without the shield.
Without the shield, $q_0=\sigma(T_1^4-T_2^4)$. At steady state each shield face transfers the same q, so $q=\sigma(T_1^4-T_s^4)=\sigma(T_s^4-T_2^4)$. Adding gives 2q = q₀, hence **q/q₀ = 1/2, A**.
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