How much of AQA A-Level Physics is module 3.3 Waves?

Waves is a core module examined mainly on Paper 1, and its superposition ideas link directly to required practicals on stationary waves and the double slit. It mixes recall (definitions of phase, coherence, polarisation and the differences between wave types) with a steady stream of calculations using the wave equation, the double-slit and grating equations, and Snell's law.

What is the difference between a stationary and a progressive wave?

A progressive wave transfers energy along its direction of travel, and every point has the same amplitude but a continuously changing phase. A stationary wave stores energy rather than transferring it, has fixed nodes (zero amplitude) and antinodes (maximum amplitude) half a wavelength apart, and all points between two nodes oscillate in phase. A stationary wave forms when two progressive waves of the same frequency travel in opposite directions and superpose.

What does coherence mean and why does it matter for interference?

Two sources are coherent if they have the same frequency and a constant phase difference. Coherence is needed to see a stable interference pattern: if the phase difference drifts, the bright and dark fringes shift around and average out, so no clear pattern is visible. This is why Young's experiment splits one source into two, and why lasers give the clearest fringes.

When does light bend towards or away from the normal, and when is it totally internally reflected?

Light entering a denser medium (higher refractive index) slows down and bends towards the normal; entering a less dense medium it bends away. Total internal reflection only happens going from denser to less dense, when the angle of incidence exceeds the critical angle given by sin of the critical angle equals the ratio of the two refractive indices. This is the principle behind optical fibres.

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AQA A-Level Physics 3.3 Waves: a complete overview of progressive and stationary waves, interference, diffraction and refraction

A deep-dive AQA A-Level Physics guide to module 3.3 Waves. Covers progressive and stationary waves, the wave equation, transverse and longitudinal waves and polarisation, superposition and coherence, Young's double-slit experiment, single-slit and grating diffraction, and refraction with total internal reflection, including the equations and exam patterns AQA repeats.

Generated by Claude Opus 4.820 min read3.3Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What module 3.3 actually demands
Progressive waves
Stationary waves
Interference and coherence
Diffraction
Refraction and total internal reflection
How module 3.3 is examined
Check your knowledge

What module 3.3 actually demands

Waves is one of the most calculation-rich early modules in AQA A-Level Physics. It builds from the description of a single progressive wave, through the superposition effects of stationary waves, interference and diffraction, and finishes with refraction and total internal reflection. The examiners test clear definitions (phase, coherence, polarisation, nodes and antinodes) alongside a steady stream of equation-based calculations.

This guide walks through the five topics of the module in specification order, then sets out the exam patterns AQA repeats. Each topic has a matching dot-point page with practice questions; this overview ties them together.

Progressive waves

The module opens with the progressive wave, which transfers energy without transferring matter. You describe it with amplitude, wavelength, period, frequency and phase, and link them with the wave equation $v = f\lambda$ and $f = \frac{1}{T}$ . Transverse waves oscillate perpendicular to the direction of travel and can be polarised; longitudinal waves oscillate parallel to it and cannot. The fact that light can be polarised proves it is transverse.

Stationary waves

A stationary wave forms when two progressive waves of the same frequency travel in opposite directions and superpose, usually a wave and its reflection. It has nodes (zero amplitude) and antinodes (maximum amplitude) half a wavelength apart, stores rather than transfers energy, and resonates on a string at $f_n = \frac{nv}{2L}$ . Contrasting stationary and progressive waves is a predictable exam question.

Interference and coherence

Interference rests on the principle of superposition: the resultant displacement is the sum of the individual displacements. Constructive interference needs a path difference of $n\lambda$ ; destructive needs $(n + \tfrac{1}{2})\lambda$ . A stable pattern requires coherent sources (same frequency, constant phase difference). Young's double-slit experiment demonstrates this with fringes spaced $w = \frac{\lambda D}{s}$ , which can be used to measure wavelength.

Diffraction

Diffraction is the spreading of waves through a gap, greatest when the gap is about the size of the wavelength. A single slit gives a wide central maximum with dimmer side maxima; in white light the centre is white and the outer fringes spread into colours. A diffraction grating has many slits, giving sharp, bright maxima at angles obeying $d\sin\theta = n\lambda$ , used to produce spectra and measure wavelengths precisely.

Refraction and total internal reflection

Refraction is the change of direction at a boundary caused by a change in wave speed, described by the refractive index $n = \frac{c}{c_s}$ and Snell's law $n_1\sin\theta_1 = n_2\sin\theta_2$ . Beyond the critical angle ( $\sin\theta_c = \frac{n_2}{n_1}$ ), light undergoes total internal reflection, the principle that guides light along optical fibres.

How module 3.3 is examined

A typical AQA profile for waves:

Definitions and explanations. Phase difference, coherence, polarisation, nodes and antinodes, and the differences between wave types.
Calculations. The wave equation, harmonic frequencies on a string, double-slit fringe spacing, the grating equation, refractive index, Snell's law and the critical angle.
Practical questions. Stationary waves on a string, the double slit and grating, and measuring refractive index, all drawn from the required practicals.

Check your knowledge

A mix of recall and calculation questions covering module 3.3. Attempt them, then check against the solutions.

State the wave equation and define each symbol. (2 marks)
Explain why only transverse waves can be polarised. (2 marks)
State two differences between a stationary and a progressive wave. (2 marks)
A string of length $0.50 \text{ m}$ carries waves at $200 \text{ m s}^{-1}$ . Find the first harmonic frequency. (2 marks)
State the path-difference conditions for constructive and destructive interference. (2 marks)
In a double-slit experiment $\lambda = 600 \text{ nm}$ , $D = 2.0 \text{ m}$ and $s = 0.30 \text{ mm}$ . Find the fringe spacing. (2 marks)
A grating has $500$ lines per mm. Find the slit spacing and the first-order angle for $\lambda = 600 \text{ nm}$ . (3 marks)
Glass has refractive index $1.52$ . Find the critical angle for a glass-air boundary. (2 marks)

Solutions

Q1: $v = f\lambda$ : $v$ is wave speed, $f$ is frequency, $\lambda$ is wavelength.
Q2: Only transverse waves have oscillations perpendicular to the direction of travel, which can be restricted to a single plane; longitudinal oscillations are along the direction of travel and have no such plane.
Q3: A stationary wave stores energy while a progressive wave transfers it; a stationary wave has nodes and antinodes (varying amplitude) while a progressive wave has the same amplitude everywhere.
Q4: $f_1 = \frac{nv}{2L} = \frac{1 \times 200}{2 \times 0.50} = 200 \text{ Hz}$ .
Q5: Constructive: path difference $= n\lambda$ . Destructive: path difference $= (n + \tfrac{1}{2})\lambda$ .
Q6: $w = \frac{\lambda D}{s} = \frac{600 \times 10^{-9} \times 2.0}{0.30 \times 10^{-3}} = 4.0 \times 10^{-3} \text{ m} = 4.0 \text{ mm}$ .
Q7: $d = \frac{1 \times 10^{-3}}{500} = 2.0 \times 10^{-6} \text{ m}$ . $\sin\theta = \frac{n\lambda}{d} = \frac{1 \times 600 \times 10^{-9}}{2.0 \times 10^{-6}} = 0.30$ , so $\theta = 17.5^\circ$ .
Q8: $\sin\theta_c = \frac{n_2}{n_1} = \frac{1.00}{1.52} = 0.658$ , so $\theta_c = 41.1^\circ$ .

Sources & how we know this

AQA A-level Physics (7408) specification — AQA (2017)