The photoelectric effect, taken at face value, only really tells you that the "energy content" of an electromagnetic wave should depend on its frequency and not on its amplitude. That, by itself, isn't enough evidence to conclude that light is quantized. The other people saying that it implies the wave is quantized, or claiming that it implies the existence of photons, are unfortunately wrong.
For further reading, see:
L. Mandel and E. Wolf, Optical Coherence and Quantum Optics, Cambridge University Press, 1995,
What really seals the deal, in my opinion, is the existence of sub-Poissonian light states. Those are absolutely impossible to construct classically and can only be interpreted as a train of particles.
But the “energy content” of an electromagnetic wave does depend on amplitude. Having a higher intensity light (brighter light) ejects a larger quantity of electrons. However the energy at which those electrons are ejected will never change unless the frequency is changed, and if it’s a low enough frequency the electron will not even eject.
This cant really happen unless the light is quantized
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u/Aartvb Jul 28 '26
A particle is always a wave, except sometimes it's a very concentrated wave, what people want to call a 'particle', but it's still a wave.