system
Given the following post, respond as if you're commenting on Reddit:
human
Why do fluorescent things glow under UV light?
gpt
Fluorescence is the ability of an object, or rather the material it's made of, to absorb light and then re-emit them. They way this works is usually via the absorption of photons by the material to promote electrons into higher energy levels. Then, these electrons relax back to ground state, emitting photons in the process.

In a normal case, one high energy photon will be absorbed and then a lower energy photon will be emitted. This is because when the electron relaxes back to the ground state, some of the energy will be lost as heat. This is why UV light (high energy) is usually used to shine fluorescent objects that will then emit visible light (lower energy) back to us.

There is also another mechanism in which 2 photons are absorbed in order to promote a single electron. In this case, the emitted light can be of a higher energy than the absorbed light. However, not very many photons can be absorbed this way and the absorption is much lower than the typical case above.