What is the photoelectric effect?
When light of a high enough frequency shines on a metal surface, electrons are emitted from the metal. These emitted electrons are called photoelectrons. The effect shows that light behaves as particles (photons), not just as waves.
Key observations
- Emission happens only if the light frequency is above a certain minimum, the threshold frequency (f₀).
- Below f₀, no electrons are emitted no matter how bright (intense) the light is.
- Above f₀, increasing the intensity increases the number of photoelectrons per second, but not their maximum energy.
- Increasing the frequency increases the maximum kinetic energy of the photoelectrons.
Key formula / 关键公式
Work function (minimum energy to release an electron):
W₀ = hf₀
where h = 6.63 × 10⁻³⁴ J s and f₀ = threshold frequency.
Worked example / 例题
A metal has a threshold frequency of 6.0 × 10¹⁴ Hz. Find its work function.
W₀ = hf₀ = (6.63 × 10⁻³⁴)(6.0 × 10¹⁴) = 4.0 × 10⁻¹⁹ J.
Why waves cannot explain it
The wave theory predicts that any light, if bright enough, should eventually free electrons — but experiments show that light below f₀ never does. This "threshold" behaviour is explained only by the photon (quantum) model, where one photon gives its whole energy to one electron.
Remember / 记住
- No emission below the threshold frequency f₀.
- Work function W₀ = hf₀.
- Brighter light gives more electrons; higher frequency gives faster electrons.