Photoelectric Effect
The photoelectric effect is the ejection of electrons from a material when light falls on it. Its importance is not the emission itself but the pattern of the emission: the maximum kinetic energy of the ejected electrons depends on the frequency of the light and not on its intensity, and below a threshold frequency no electrons come out at all, however bright the source.
The standard account
Hertz noticed in 1887 that a spark gap fired more readily when illuminated with ultraviolet light. Philipp Lenard established the key facts by 1902: the number of electrons emitted scales with intensity, but their maximum energy does not; that energy rises linearly with frequency; and emission begins essentially immediately, without the delay a classical wave depositing energy gradually would require.
Albert Einstein proposed in 1905 that light is absorbed in discrete quanta of energy hf, giving
- Kmax = hf − φ
where φ is the work function, the energy needed to free an electron from the material, and h is Planck's constant. The relation is a straight line of slope h with intercept −φ. Robert Millikan, who set out to disprove it, spent a decade on the measurement and in 1914–1916 confirmed the linear relation and obtained a value of h from its slope. Einstein's Nobel Prize in 1921 was awarded specifically for this work, not for relativity.
One qualification belongs in any honest statement of the standard account. The photoelectric effect on its own does not prove that the electromagnetic field is quantized. In the semiclassical treatment developed by Lamb and Scully and others, a classical wave incident on a quantized atom reproduces the threshold, the linearity in frequency and the prompt emission, because the discreteness lies in the detector rather than in the light. Field quantization rests on later evidence — photon antibunching, sub-Poissonian photon statistics, and single-photon interference experiments. This is not a dissident claim; it is standard quantum optics, and it is the technically correct reason why "the photoelectric effect proves the photon exists" is an overstatement found in textbooks.
On this wiki
That overstatement is exactly the target of most of the work catalogued here. This wiki holds a substantial body of papers proposing classical, wave or aether-based accounts of photoelectric emission, and most of them are attacking the inference from the effect to the photon rather than the data.
- Bing-Xin Gong, "A Classical Approach to the Photoelectric Effect & Photoelectron Emission" — a classical derivation of the emission characteristics.
- Johann Marinsek, "Physics EA: Photoelectric Effect No Empirical Confirmation for the Existence of a Photon That Carries Energy Packet E = hf" — the most direct statement of the inferential objection, and the one that lands closest to the semiclassical point above.
- Thomas Smid, "Wave and Particle Theory of Light applied to the Photoelectric Effect" — compares the two treatments against the same data.
- Ionel Dinu, "Radio Waves - Part III: The Photoelectric Effect" — part of a series treating light and radio emission on a single continuous wave picture.
- Robert H Schor, "Einstein and the Photoelectric Effect" — on the historical and logical status of the 1905 argument.
- Charles William Lucas, "A Physical Model for Atoms and Nuclei, Part 4: Blackbody Radiation and the Photoelectric Effect" — derives both the photoelectric relation and the blackbody spectrum from the extended-particle electrodynamics of Common Sense Science, in which the electron is a toroidal ring rather than a point. Lucas treats the two effects as one problem, which is the right instinct: they stand or fall together as evidence for quantization.
The photoelectric effect, the Compton Effect and Blackbody Radiation are the three pillars on which the photon is usually erected, and this collection contains alternatives to all three. Readers comparing them should keep the distinction above in view: showing that a classical model can reproduce the photoelectric data is a real result, and several of these papers achieve something like it, but it does not by itself dispose of the photon, because the photon's modern evidential base lies elsewhere.