Photoelectric equation
(Q2.2.1) Q2.2 · The Photoelectric Effect →The maximum kinetic energy of an ejected electron, from the photon energy hf minus the metal's work function φ.
| maximum kinetic energy of ejected electrons | J | |
| Planck constant | 6.626×10⁻³⁴ J·s | |
| light frequency | Hz | |
| work function | J |
Light striking a metal surface, with hf compared directly against that metal's own work function.
Below the threshold frequency (hf < work function), no electrons are ejected regardless of light intensity — the formula predicts a negative, unphysical kinetic energy in that regime and simply doesn't apply.
Einstein proposed light itself is quantized into photons of energy hf; an electron absorbs one photon whole, escaping only if that energy exceeds the work function that binds it to the metal:
K_max = hf − φ contains no intensity term at all:
Doubling the light's intensity doubles the number of photons arriving per second, and so the number of ejected electrons — but leaves the energy of each individual electron completely unchanged.
Below the threshold frequency (hf < φ), no electrons are ejected however intense the light. Intensity affects the NUMBER of ejected electrons per second, not the energy of each one — the historically crucial observation that proved light behaves as particles.