In the photoelectric effect, we observe that the stopping potential is independent of the intensity of the light. This is readily explained by the photon hypothesis. One often sees the statement that in "the classical theory," the stopping potential should increase with intensity. What classical theory does this refer to? If people mean the 1924 Bohr-Kramers-Slater theory, there are several problems: (1) it would be odd to refer to it as "the classical theory," when it is actually a hybrid classical-quantum theory; (2) it seems anachronistic to invoke it when discussing Einstein's 1905 photon hypothesis; and (3) BKS was disproved in 1925 by Bothe. If they really do mean some specific classical theory, then what theory is it? Classically a system of electrically charged particles can't form stable bulk matter, but let's hand-wave that issue away. I would expect that at any particular temperature, the electrons would have a Maxwellian velocity distribution. There would be some rate of emission from the cathode, which would depend on temperature. A visible light wave impinging on the cathode has a wavelength at least a thousand times greater than the size of an atom, so it can't affect one atom without affecting many others in the area equally. I would expect the effect to be simply a warming of the cathode, and the amount of warming would depend on things like the intensity of the light and the ability of the cathode to get rid of its heat. Under ordinary conditions, this warming would be negligible. In any case, there would be no well-defined stopping voltage. Electrons would come out of the cathode with some continuous velocity distribution shaped like the tail of a Maxwellian distribution. I would like to avoid having this thread sidetracked by a discussion of the claim by Lamb and Scully that the photoelectric effect can be explained without photons. I think their argument is bogus, for reasons given here: http://physics.stackexchange.com/questions/68147/can-the-photoelectric-effect-be-explained-without-photons . In any case, they're invoking BKS, and BKS seems irrelevant to me for the reasons described above.