eli5: Why can’t the photoelectric effect be explained by the wave theory of light?
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Basically, the photoelectric effect is a discrete phenomenon, i.e. it involves separete states, not a continuous change.
The photoelectric effect consists in light hitting an atom and a consequence, one of its electrons gains energy and enters an excited state.
This happens only when the light is above a certain frequency, and the wave model of light does not have a good explaination for this. A wave with a higher frequency carries more energy, but so a wave with a greater amplitude. You would expect that a wave with lower frequency and greater amplitude, which overall carries more energy could also trugger this effect, but it dows not happen.
On the other hand in particle model, the energy of the light is carried through discrete packages named photons. An higher frequency corresponds to more energy per photon, while greater amplitude corresponds to a greater flow rate of photons.
This model is compatible with the photoelectric effect because the atoms absorbs only one photon at the time, and so it explains why a greater umber of individually weaker photons does not trigger it.
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