3.10 Photoelectric Effect

What is the photoelectric effect?

The photoelectric effect occurs when light of sufficient energy strikes a metal surface, ejecting electrons. This shows light behaves as particles called photons, each with energy E = hν, where h is Planck's constant and ν is frequency. Only light above a threshold frequency can eject electrons, no matter how intense the light is.

Energy and photoelectrons

Each photon transfers its energy to one electron. If the photon energy exceeds the binding energy (ionization energy) holding the electron, the excess becomes kinetic energy of the ejected electron. Increasing light intensity increases the *number* of ejected electrons, not their individual energy — only increasing frequency increases kinetic energy per electron.

Connecting to atomic structure

Photoelectron spectroscopy (PES) applies this principle: electrons in different energy levels/sublevels require different amounts of energy to remove. Measuring the kinetic energy of ejected electrons reveals the ionization energies of electrons in various shells, giving evidence for quantized electron energy levels within atoms.

Key terms

Photoelectric effect

Emission of electrons from a metal surface when struck by light of sufficient frequency.

Photon

A discrete packet of light energy; E = hν, where h is Planck's constant and ν is frequency.

Threshold frequency

The minimum light frequency needed to eject an electron from a given metal; intensity alone can't substitute for it.

Effect of increasing intensity (above threshold)

Ejects more electrons per second, but does not increase each electron's kinetic energy.

Photoelectron spectroscopy (PES)

Technique measuring kinetic energy of ejected electrons to determine ionization energies and confirm quantized energy levels in atoms.

Try an AP-style question

Red light doesn't eject electrons from a metal, but UV light does. Why?

1. Red light is absorbed by the air before reaching the metal
2. Red light contains more photons than UV light
3. Only UV photons have enough energy to exceed the metal's binding energy
4. UV light is always brighter, so it hits harder