A level Physics · Edexcel 9PH0 · Topic 5E

The photoelectric effect

Key points

The photoelectric effect: electrons are emitted from a metal surface when light above a certain frequency shines on it.
Observations: no emission below the threshold frequency $f_0$ , however bright the light; emission is instant; the maximum kinetic energy of electrons depends on frequency, not intensity.
The wave model cannot explain this; the photon model can. Light comes in packets of energy $E = hf$ , and one photon gives all its energy to one electron.
Work function $\phi$ is the minimum energy needed to free an electron from the surface: $\phi = hf_0$ .
Einstein's photoelectric equation: $hf = \phi + \frac{1}{2}mv_{max}^2$ .
Greater intensity means more photons per second, so more electrons per second (larger current) but the same maximum kinetic energy.
The electronvolt is the energy gained by an electron moving through 1 V: 1 eV = $1.6 \times 10^{-19}$ J.

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Shining light on a metal can release electrons, but the way this happens cannot be explained if light is only a wave. The photoelectric effect is the key evidence that light energy arrives in packets called photons.

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