Why can't energy is absorbed by protons rather than by electrons when a light ray strikes the atom.
Light is absorbed when there is an excited state to go to.
You can picture a proton as the ground state of the (uud) 3-quarks system. The first excited state of this system (if it exists at all… I don't know…) is very high in energy and unreachable by photons with “reasonnable” energies
Protons can absorb photons too but it's easier for an electron to absorb it because it can move around
Nuclear energy levels are measured in MeV (millions of electron volts), whereas photons of visible light have energies of only a few eV.
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