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It seems that Einstein's 1905 paper "Concerning an Heuristic Point of View Toward the Emission and Transformation of Light" notes the discrete quanta of light energy, an idea that leads to thinking of light as consisting of "particles". What experiments and/or understanding exclude the possibility that photons are simply waves whose source involves a process which is discrete in nature, only producing waves at a discrete energy level?

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  • $\begingroup$ What does "whose source involves a process which is discrete in nature, only producing waves at a discrete energy level" mean? How is that different from quantum theory? It is quite often that people make such statements, but often by just studying these two questions, they would either realise they were led back to quantum theory, or that their non-quantum guesses have already been rejected by experiments. $\endgroup$ Commented Apr 20, 2023 at 15:31

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Most obvious particle-like property of photons: Photons can be counted.

Related:
Particle- and wave-like properties
How does quantization arise in quantum mechanics? (for more clarity about what photons really are.)

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Einstein tried to explain the photo-electric effect, where light at a certain wavelength (color) knocked-off electrons from the photo-electric material, but light at another wavelength didn't. Both are waves, and both can have the same "energy level" if the light intensity is made strong enough. So to explain why one wavelength knocked off electrons and another (even if shone at very high intensity) didn't, one needs to assume that these waves are somehow divided into "packets" and to knock of an electron you need an individual packet with enough energy - and it's not enough to have a thousand of smaller-energy packets.

But why would the energy in each of these packets be proportional to the frequency (1/wavelength) of the wave, and not something else? This specific energy content of these "quanta" had already been suggested five years earlier, by Max Planck, to explain the spectrum of black-body radiation without falling victim to the "ultraviolet catastrophe".

These energy packets are exactly what we call today "photons".

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If the w absorption and emission of light wasnt done by a particles then a single EM wave would have been able to excite simultaneously 2 or more electrons which simply isnt observed.

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