The experimental observation(s) below that require(s) a quantum explanation for the photoelectric effect
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is that more photoelectrons are emitted when the light frequency increases.
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is that the maximum kinetic energy of the photoelectrons is related linearly to the frequency of the light.
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is that every metal surface has a work function, a minimum amount of energy needed to free electrons.
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is that the stopping potential measures the kinetic energy of the photoelectrons.
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are all of the above.