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.
It takes a certain amount of energy to kick an electron out of the atom, thereby producing electricity. The energy of light is correlated wavelength. Shorter wavelength light has more energy, longer has less. In a wave theory of a lot of low wavelength light would add up to be enough kick electrons out of the atom. But this isn’t what happens. The light must be of a sufficient energy level to before any electrons are releases. For example a huge amount of infra-red light might not be able to produce electricity, but a small amount of ultraviolet would be able to. This proves that the energy is light is in discreet packets, not continuous waves.
I read a great explanation for this effect. Imagine that there are a bunch of people being held in jail. If they can pay $100 they will be released. If you go and drop dollar bills into the jail, they will be spread out among the inmates and no single person will have enough to get out. It doesn’t matter if you increase the number of dollar bills that you drop into the jail because there are so many inmates that no one gets the critical amount to get out.
But, if you start dropping $100 bills, each bill will pay the way for a single person and people will start getting out.
The $1 bills are like low frequency light. It doesn’t matter how bright the light is, it won’t trigger the photoelectric effect. But, if you shine higher frequency light at the cell, each individual photon has enough energy to knock an electron off and you will start seeing current flow.
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