system
Given the following post, respond as if you're commenting on Reddit:
human
Where does the energy difference between absorbed and emitted photons go to?

My teacher told me when a photon is absorbed and emitted again, it has a lower frequency. So it loses energy. Where does that energy go to?
gpt
Typically, when we talk about photons being absorbed and emitted, we're talking about electrons in an atom doing the absorbing and emitting. An election in an atom is attracted to the positively charged nucleus, which causes it to "orbit" in the closest possible position to the nucleus. (I put orbit in quotes because electrons don't literally orbit like planets, but it's a useful image.)

In order to move an electron away from its nucleus, you need to add energy to it. It's similar to pulling two magnets apart: if you don't touch the magnets, they'll stay connected, but you can pull them apart, which uses the energy in your body and adds potential energy to the magnets (the potential energy turns into kinetic energy when you let go of the magnets and they snap back together).

So when a photon is absorbed by an electron, the electron gains the photon's energy, causing it to jump to an orbit further away from the nucleus. If the photon has enough energy, it can even knock the electron clean off the atom.

Now, due to quantum mechanics, an electron can only orbit at certain distances around an atom, and these specific distances correspond to certain amounts of energy. After a little bit of time, an electron that has been bumped up to a higher energy orbit will re-emit a photon, losing some energy and jumping back down to a lower-energy orbital. Most of the time, it will re-emit a photon of the same energy that it absorbed and jump back to the same energy level it was at before. But if the photon was energetic enough to knock the electron up by a few energy levels, it is possible for the electron to only drop partway, and release a photon with less energy. That photon will have the energy equal to the difference of the two orbitals that the electron moved between. A short time later, the electron will drop again, emitting *another* photon with less energy than the original, again equal to the difference in energy between the electron's old orbital and it's new one. This process can happen as many times as there are empty orbitals between the electron's current orbital and the one it wants to get back to (which is the lowest-energy orbital that is currently unoccupied), but usually an electron just drops the whole way in one step, and less frequently drops in smaller steps.

In any event, when an electron drops some number of steps at a time back to its minimum energy orbital, the energy of all the photons it emitted will add up to equal the energy of the original photon that it absorbed!