I read that good absorbers are good emitters – hence a blackbody, that absorbs all kinds of radiation, also emits all kinds of radiation? I'm not able to get my head around this.
What does it mean to absorb all kinds of radiation? Radiation of all frequencies? or are we saying that $100\%$ radiation incident on a blackbody is absorbed? None is reflected? Well then, how is it a good emitter?
Am I confusing good emitter and good reflector? I'd appreciate some clarifications. Thank you!
P.S. I'm coming back on Physics SE after years. I'm majoring in mathematics, but I decided to take a quantum physics course for fun anyway. Hence, I'm back here for the spring!
Best Answer
Yeah, good emitter and good reflector are definitely not the same property. Maybe the best way to visualize it is just as the time inverse, under the mapping $t\mapsto-t$, of absorption is emission. This also gives something of a hint of why this relationship might hold, albeit I only know the derivation for the case of thermal radiation.
For thermal radiation, you bring two bodies of the same temperature into radiative contact, one of one material, one of the other. If either one emits more radiation than it absorbs, then it spontaneously cools and the other one spontaneously heats up, violating the second law of thermodynamics. Very simple argument. May require sealing the two in some idealized setup of mirrors or so, to make the proof work, but aside from that it appears to have immediate physical relevance.