eli5 what determines if a photon hits an electron?
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Well it depends on what you mean by hitting. Lets say that a photon comes by an electron the electron absorbs it and reemits it. For that the wavefunction of the photon has to “collapse” when the electron comes by. The light exists as a wave and if it happens to encounter the electron it turns into a photon. The photon is more of an event rather than a tiny ball.
So nothing really determines it as its probabilistic, its indeterministic as all things in quantum mechanics. But it does depend on the energy of the photon.
If you have an electron like in an atom it depends where that electron is temperature etc, but you need the photon to have a certain energy. Here you might need to consider relativistic effects because the electrons that move towards the light see that the photon has more energy. Those that move away from the light see it with less energy.
Thats actually a way to cool gases to very small temperatures. You shine some low energy light on it and if the electrons/atoms didn’t move they would absorb it. When this absorption emission happens the electrons/atoms gets pushed back a bit. So if you are an atom and move towards the light you see the light to have more energy so you absorb it and get pushed back so you slow down. If you aren’t flying towards the light nothing will happen. You shine a box on all sides with this light and it will slow the atoms down to near 0.
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