# Why would light not heat a body if the source of the light is the same temperature as that body?

I'm specifically looking at the case of the Sun's corona emitting light that then heats up other bodies on Earth. These bodies can never attain a higher temperature than the Sun's corona.

I know the obvious answer as to why this is is that the second law of thermodynamics says so. But the fact that the light emitted somehow encodes within it the temperature of the thing that emits it is what is strange to me. Are there not bodies of different temperatures that emit the same frequencies of light? Is there some deeper set of statistical mechanics taking place such that the second law of thermodynamics holds?

It makes sense for bodies directly interacting to end up at the same temperature, and at the microscopic scale this would seem to happen because of electrostatic interactions between the bodies. Whichever atom in an atom-atom interaction has a greater kinetic energy will impart kinetic energy on the atom with a lower kinetic energy until equilibrium. A bit of handwaving, I'm sure, because we are considering classical atom-atom interactions, but it makes quite good sense and probably isn't completely off. However, light seems to somehow encode the temperature of what emitted it, and I can't explain how the second law of thermodynamics holds with it.

Why is it that the light emitted by a body of a certain temperature will not heat up a body of the same or greater temperature? Answers I'd like would sort of explain the interactions taking place such that the second law makes some sort of sense, perhaps with Newton's laws, statistical mechanics, Maxwell's equations... whatever else makes sense would be appreciated, really.

• Is that truly the case? Any source? I think using a lens large enough, you'll be able to obtain local temperatures higher than at the source.
– SF.
Commented Dec 22, 2016 at 13:34
• "the fact that the light emitted somehow encodes within it the temperature of the thing that emits it is what is strange to me." That is what the black body radiation spectrum represents. Commented Dec 22, 2016 at 14:07
• @SF. Doesn't your statement violate second law?
– Deep
Commented Dec 23, 2016 at 5:16
• @Zero: Entropy in the whole system increases. Localized decreases of entropy are not violating second law of thermodynamics. (otherwise existence of life would be violating it)
– SF.
Commented Dec 23, 2016 at 15:44
• @SF. I asked that only because your statement sounds like it is possible to transfer heat from colder to hotter body, without any external input of work.
– Deep
Commented Dec 24, 2016 at 14:16