Why can’t an object become hotter than the light source that’s illuminating it?
In: Physics
I would think that it has to do with the law of conservation of energy which says the the total energy in a closed system .use remain constant. So if one thing is giving off some energy and something else is absorbing that energy, then the object receiving energy can’t get hotter then the one giving energy.
I’ll assume for a moment that your light source here is glowing due to heat (like the sun or an old light bulb). Something like a laser can make other things hot without becoming hot itself, because it generates light through other processes. (It still consumes energy, though, which is why the next bullet point still applies.)
The simplest answer is that if it happened without input of energy you’d be able to extract free energy from such a process (by running a heat engine off of the difference), and thermodynamics says you can’t do that.
If you’re talking about lenses, see [this discussion from xkcd](https://what-if.xkcd.com/145/), which gets into some high-school-physics level details of why that doesn’t work.
If you’re just talking about an object sitting passively in light: because objects emit energy in the form of a glow, which rapidly scales up the hotter they are. When the illuminated object gets close to the temperature of the light source, it’s emitting so much energy of its own that it’s releasing as much energy as is coming in.
otherwise the object would heat up the light source.
The net energy flow is determined by temperature *difference*.
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