The photoelectric effect
Light striking a metal surface ejects electrons only if a single photon carries enough energy to free one, no matter how bright the light is — evidence that light behaves as particles, not just waves.
Practise this conceptShine light on a metal surface and, under the right conditions, it kicks electrons out — but "the right conditions" turned out to be far stranger than anyone expected before Einstein explained it in 1905.
Below a certain threshold frequency, no electrons come out at all, no matter how intensely bright the light is turned up. This flatly contradicted the wave picture of light, which predicted that a bright enough dim-frequency beam should eventually deliver enough energy to free an electron. Photons resolve the puzzle: each photon either carries enough energy on its own to free an electron, or it doesn't, and piling on more low-energy photons per second (more brightness) never adds up to one photon crossing that threshold.
Above the threshold frequency, increasing the light's intensity does increase the number of electrons emitted per second, but not their individual maximum kinetic energy — that depends only on the light's frequency and the metal's work function, evidence that a single photon transfers its energy to a single electron in one discrete event, not gradually over time.
Key ideas
Requires: Photons and quanta
Unlocks: Quantised energy levels in atoms
Common misconceptions
- Increasing the brightness of light below the threshold frequency will eventually cause electrons to be emitted.
- The kinetic energy of emitted electrons depends on the intensity of the light.
- The photoelectric effect can be fully explained by treating light purely as a wave.