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The photoelectric effect

When light hits a metal, a photon can knock out an electron if the photon's energy is greater than the work function. The surplus becomes kinetic energy of the electron. Stronger light gives more photons and therefore more electrons, but not more energy per electron. It is the frequency, or the wavelength, that sets the energy per photon. Red light can therefore have no effect on a metal from which ultraviolet light does release electrons.

E≈1240λE \approx \frac{1240}{\lambda}photon energy in eV when λ is in nm
Ek=E−WE_k = E - Wmaximum kinetic energy

Symbols

EEphoton energyeV
λ\lambdawavelengthnm
WWwork functioneV

Example

Light with wavelength 400 nm hits a metal with work function 2.3 eV.

E≈1240/400=3.1E \approx 1240/400 = 3.1 eV and Ek=3.1−2.3=0.8E_k = 3.1 - 2.3 = 0.8 eV.

If the photon energy is less than the work function, no electrons are released, however strong the light is.
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Part of Physics 2: Quantum physics and relativity.