# Photons and the photoelectric effect

> The photon model, the photoelectric equation, work function and threshold frequency, wave–particle duality and de Broglie wavelength, and atomic energy levels.

Canonical: https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.quantum
Last updated: 2026-08-01

Flagship subject: topics and specification statements are authored to the WJEC specification. No question in it has been through human review yet.

- Subject: [WJEC · A Level · Physics](https://revise-gray.vercel.app/library?subject=wjec-alevel-physics)
- Specification reference: Unit 2.5
- Difficulty: 4/5
- Assessment objectives: AO1, AO2
- Status: authored · checked · 2026-08-01

## Specification coverage

| Reference | Statement | AOs |
| --- | --- | --- |
| Unit 2.5(a) | use the photon energy relation and the photoelectric equation | AO1, AO2 |
| Unit 2.5(b) | explain why no emission occurs below the threshold frequency regardless of intensity | AO2, AO3 |
| Unit 2.5(c) | describe how photon intensity affects number, not energy, of emitted electrons | AO1 |
| Unit 2.5(d) | apply the de Broglie relation and describe electron-diffraction evidence for wave-particle duality | AO1, AO2 |
| Unit 2.5(e) | calculate de Broglie wavelength and interpret electron diffraction evidence | AO2 |
| Unit 2.5(f) | describe discrete atomic energy levels and emission and absorption spectra | AO1 |
| Unit 2.5(g) | apply electron volt conversions and energy level transitions | AO2 |

## Summary

The photon model, the photoelectric equation, work function and threshold frequency, wave–particle duality and de Broglie wavelength, and atomic energy levels.

## What earns marks

- E = hf = hc/λ, and hf = φ + ½mv²max.
- Below the threshold frequency no electrons are emitted however intense the light — this is what the wave model cannot explain.
- Increasing intensity increases the number of photoelectrons, not their maximum kinetic energy.
- de Broglie: λ = h/p, confirmed by electron diffraction.

## Where marks are lost

- Saying brighter light gives faster electrons.
- Forgetting to convert eV to joules (1 eV = 1.6 × 10⁻¹⁹ J).
- Using the work function as an energy per mole.

## Related

- [WJEC · A Level · Physics](https://revise-gray.vercel.app/library?subject=wjec-alevel-physics)
- [Photons and the photoelectric effect lesson](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.quantum)