7.1 Photoelectric Effect

Light knocks out electrons

In the photoelectric effect, light shining on a metal surface ejects electrons. This only makes sense if light behaves as particles called photons, each carrying energy E = hf, where h is Planck's constant and f is frequency. A single photon transfers all its energy to a single electron in one collision.

Threshold frequency and work function

Every metal has a work function (φ) — the minimum energy needed to free an electron. Photons below the threshold frequency (f₀) can't eject electrons, no matter how intense the light. Above threshold, extra photon energy becomes the electron's kinetic energy: KE_max = hf − φ.

Why intensity doesn't matter (but frequency does)

Classical wave theory predicted brighter light should give electrons more energy, but experiments showed increasing intensity only increases the number of electrons ejected, not their energy. Only increasing frequency increases each electron's kinetic energy. This supported Einstein's photon model over the classical wave model.

Key terms

Photoelectric effect

Emission of electrons from a metal surface when light of sufficient frequency shines on it.

Photon energy equation

E = hf, where h is Planck's constant and f is the frequency of light.

Work function (φ)

Minimum energy required to eject an electron from a metal's surface.

Threshold frequency (f₀)

The minimum light frequency capable of ejecting electrons from a given metal; below it, no electrons are emitted.

Maximum kinetic energy equation

KE_max = hf − φ, the photon energy minus the work function.

Try an AP-style question

Photons of energy 5.0 eV strike a metal with work function 2.0 eV. What is the maximum kinetic energy of the ejected electrons?

1. 10 eV
2. 7.0 eV
3. 3.0 eV
4. 2.5 eV