$(25pt)$ Consider coaxial, parallel, black disks separated

by a distance of $0\mathrm{.}20m.$ The upper disk of diameter $0\mathrm{.}20$

$m$ is maintained at $456K$ and the surroundings are at $300$

$K.$ The diameter of the lower disk is $0\mathrm{.}4m.$ What is the net

heat transfer by radiation from the lower disk? $($Emissivity

of surroundings is $=1)$