Photoelectric Effect
The photoelectric effect is the ejection of electrons from a material's surface when struck by light of sufficiently high frequency, with the ejected electrons' kinetic energy depending on the light's frequency but not its intensity — a fact classical wave theory could not explain.
Curious
Classical physics treated light purely as a wave, and waves can carry any amount of energy — just make the wave big enough (bright enough light) and eventually it should shake an electron loose, no matter the colour. Experiments showed the opposite: dim ultraviolet light ejected electrons instantly, while extremely bright red light, no matter how intense, ejected none at all from the same metal.
Einstein's 1905 explanation (not the same paper as his more famous relativity work that year, though published in the same "miracle year") proposed that light itself arrives in discrete packets — photons — each carrying a fixed energy set by its frequency, not its brightness. Brightness only controls how many photons arrive per second, not how much energy each one carries.
Exploring
See the interactive photoelectric effect simulation for a live, adjustable demonstration of the relationship below, and the Equations Library section generally for the full equation derivation.
Below the threshold frequency f₀ = Φ/h, no electrons are ejected regardless of intensity — this threshold behaviour, sharply confirmed by Robert Millikan's precision measurements (1916), was the decisive experimental evidence for Einstein's photon hypothesis and let Millikan measure Planck's constant independently, matching the value from blackbody radiation to within experimental error.
Deep Dive
Einstein's 1905 paper was explicitly cautious, describing the photon hypothesis as a "heuristic viewpoint" rather than a settled fact — a mark of how radical treating light as particulate was at the time, given the overwhelming 19th-century evidence (interference, diffraction) for light's wave nature established by Young, Fresnel, and Maxwell. Einstein received the 1921 Nobel Prize in Physics specifically "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect" — notably not for special or general relativity, which remained more contested within the Nobel committee at the time.
The photoelectric effect was one half of the evidence that led to wave-particle duality being recognised as a general feature of light and matter, with de Broglie's 1924 hypothesis providing the complementary matter-wave half. Modern quantum electrodynamics (QED) fully reconciles the photon picture with the well-established wave phenomena of classical optics, treating the electromagnetic field itself as quantised.
Concept Relationship Map
Requires: wave-particle duality
Enables: de Broglie hypothesis, quantum electrodynamics
Commonly confused with: thermionic emission; the false intuition that brighter light should always eject faster electrons
Sources
- Einstein, A. (1905). Annalen der Physik, 17(6), 132–148.
- Millikan, R.A. (1916). "A Direct Photoelectric Determination of Planck's h." Physical Review, 7(3), 355–388.
- The Nobel Prize in Physics 1921 — official citation. nobelprize.org