# Photon exchange

Assuming no conduction and no convection will a colder body be able to send photons to a hotter body while the hotter body is sending photons to t. We all know that an equilibrium will result with a temperature somewhere between the two bodies but do the photons emitted by the cooler body ever end up in the hotter body?

phinds
Gold Member
2019 Award
Assuming no conduction and no convection will a colder body be able to send photons to a hotter body while the hotter body is sending photons to t. We all know that an equilibrium will result with a temperature somewhere between the two bodies but do the photons emitted by the cooler body ever end up in the hotter body?
Yes, ALL bodies above absolute zero radiate. Warmer bodies just radiate more, so the NET for the colder body is that it absorbs more radiation than it sends out but that doesn't mean that it doesn't send any out.

jbriggs444
Homework Helper
2019 Award
We all know that an equilibrium will result with a temperature somewhere between the two bodies but do the photons emitted by the cooler body ever end up in the hotter body?
The short answer is "yes". Some of the thermal radiation from the cool body will be absorbed by the hot body.

This scenario appears to assume that there is an ideal perfectly reflective box surrounding the pair of bodies. Otherwise, the equilibrium temperature would be that of the environment. All of the thermal radiation from either body will end up being reabsorbed. Some by the one body and some by the other.

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ZapperZ
Staff Emeritus
Assuming no conduction and no convection will a colder body be able to send photons to a hotter body while the hotter body is sending photons to t. We all know that an equilibrium will result with a temperature somewhere between the two bodies but do the photons emitted by the cooler body ever end up in the hotter body?

To make this very clear, electromagnetic radiation can travel through vacuum (i.e. when conduction and convection are not possible). Infrared radiation is "heat", and it is a form of electromagnetic radiation. This is how heat from the sun reaches us.

The rest of your question has been addressed by the previous responses.

Zz.

How does the cool body know when to stop sending photons to the hot body? At equilbrium are each of them still continually sending photons to each other? Is each photon from the cool body at a lesser energy level than each one from the hot body? I don;t understand exactly how the 2 bodies end up with the same energy.

Nugatory
Mentor
How does the cool body know when to stop sending photons to the hot body?
It doesn't, because...
At equilbrium are each of them still continually sending photons to each other? Is each photon from the cool body at a lesser energy level than each one from the hot body?
At equilibrium the amount of energy flowing in each direction is the same. If there were any imbalance, then the object receiving more energy would warm up slightly, increasing the rate at which it radiated energy and reducing the imbalance. (As an aside, thinking in terms of photons instead of the intensity of the electromagnetic radiation just complicates the problem - photons aren't what you think they are)
I don't understand how the 2 bodies end up with the same energy
They don't. They end up with the same temperature.

phinds
Assuming no conduction and no convection will a colder body be able to send photons to a hotter body while the hotter body is sending photons to t. We all know that an equilibrium will result with a temperature somewhere between the two bodies but do the photons emitted by the cooler body ever end up in the hotter body?

Yes. It's just like shining a flashlight into a fire.