# Waves, superposition and optics

> Transverse and longitudinal waves, the wave equation, refraction and total internal reflection, polarisation, superposition, stationary waves and diffraction gratings.

Canonical: https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.waves
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 2.1–2.3
- Difficulty: 3/5
- Assessment objectives: AO1, AO2
- Status: authored · checked · 2026-08-01

## Specification coverage

| Reference | Statement | AOs |
| --- | --- | --- |
| Unit 2.1(a) | use the wave equation and describe how frequency is preserved on refraction | AO1, AO2 |
| Unit 2.1(b) | apply Young slits and grating equations to determine wavelength | AO2 |
| Unit 2.1(c) | explain formation of stationary waves and spacing of nodes/antinodes | AO1, AO2 |
| Unit 2.1(d) | state conditions for total internal reflection and apply the critical-angle relation | AO1, AO2 |
| Unit 2.2(a) | describe polarisation as evidence for transverse nature of light | AO1 |
| Unit 2.3(a) | explain refraction by Snell law n1 sin theta1 equals n2 sin theta2 | AO2 |
| Unit 2.3(b) | analyse two source interference including path difference for constructive and destructive | AO2 |
| Unit 2.3(c) | describe diffraction through a single slit and its effect on interference pattern | AO1 |

## Summary

Transverse and longitudinal waves, the wave equation, refraction and total internal reflection, polarisation, superposition, stationary waves and diffraction gratings.

## What earns marks

- v = fλ, and the frequency of a wave is unchanged when it refracts.
- Two-source interference: dλ = ax/D for Young's slits; nλ = d sinθ for a grating.
- Stationary waves need equal-amplitude coherent waves of the same frequency travelling in opposite directions; ideal nodes have zero displacement and are λ/2 apart.
- Total internal reflection requires light in the denser medium and an angle beyond the critical angle, sin C = 1/n.

## Where marks are lost

- Saying wavelength stays the same on refraction (it is frequency that does).
- Mixing up slit separation and grating spacing d = 1/N.
- Describing a node as a point of maximum displacement.

## Related

- [WJEC · A Level · Physics](https://revise-gray.vercel.app/library?subject=wjec-alevel-physics)
- [Waves, superposition and optics lesson](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.waves)