Consider a plane sheet of material or absorptivity a, emissivity e, and reflectivity r = 1- a. Let the sheet be suspended between and parallel with two black sheets maintained at temperatures τu and τi,. Show that the net flux density of thermal radiation between the black sheets is (1 – r) limes the flux density when the intermediate sheet is also black as in Problem 8, which means with a = e = 1; r = 0. Liquid helium dewars are often insulated by many, perhaps 100, layers of an aluminized Mylar film called Super insulation.