Imagine two parallel, (infinite) planar sheets of perfectly black material, maintained at fixed temperatures th and tl, respectively, and separated by some finite distance d. suspended in between and parallel to these sheets are n additional, separate, parallel sheets of material, all with constant reflectivity r. (by separate, we mean that none of the sheets are in actual mechanical contact). when the entire system reaches a steady state, find (a) the net flux density (energy flow per unit time per unit area) of thermal radiation between the original two sheets. (b) how does this compare to the radiation flux that would be present between the two black sheets in the absence of the n intervening sheets? (c) to provide the best insulation, should these intermediate sheets be made of very shiny or very black material, or something in between?