Hey, I was watching the Khan Academy India video on Wien's Displacement Law and I'm kind of confused. At the beginning, it says a black body absorbs literally everything, like all light and heat. But then in the experiment, when he heats the black body up, it starts emitting white light! How is that possible if it’s supposed to just absorb everything and not let any light out?
[disclaimer -- it's been 35 years since my last college physics class, though I did refresh my understanding by looking at science websites]
I can understand your confusion. It's not that a black body doesn't emit light, it's that it doesn't reflect light.
As I understand it, an idealized black body absorbs all electromagnetic radiation (light) that reaches it, and emits thermal radiation based solely on temperature. For a given amount of energy, the light emitted by the black body does not depend on the wavelengths of light shining on it.
Let's contrast that to ordinary objects. A pomegranate will reflect red wavelengths of light, but only if red light reaches it -- if only blue light shines on it, it will look black. A blueberry will reflect blue light if there is some, but appear black in red light.
Does that make sense?
~ Janice, KA user
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