464 Introduction to Quantum Physics(b)The open slots between the glowing tubes are brightest. When you look into a slot, youreceive direct radiation emitted by the wall on the far side of a cavity enclosed by the fixture;and you also receive radiation that was emitted by other sections of the cavity wall and hasbounced around a few or many times before escaping through the slot. In Figure 40.1 in thetext, reverse all of the arrowheads and imagine the beam getting stronger at each reflection.Then the figure shows the extra efficiency of a cavity radiator. Here is the conclusion ofKirchhoff’s thermodynamic argument: ... energy radiated. A poor reflector—a goodabsorber—avoids rising in temperature by being an efficient emitter. Its emissivity is equalto its absorptivity: ea=. The slot in the box in part (a) of the question is a black body withreflectivity zero and absorptivity 1, so it must also be the most efficient possible radiator, toavoid rising in temperature above its surroundings in thermal equilibrium. Its emissivity in
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