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Detailed notes on Waves for Cambridge International A Levels Physics, covering key concepts, explanations, examples, and exam-focused revision points.
Two fundamental wave classes. Light vs sound. Polarisation possibility.
Mapped to the Cambridge International A Level 9702 syllabus (2025-2027).
Perpendicular or parallel oscillation.
Transverse. Particle oscillation PERPENDICULAR to direction of wave travel.
Longitudinal. Particle oscillation PARALLEL to direction of wave travel.
Compressions and rarefactions are the longitudinal wave's equivalent of crests and troughs.
Cambridge tip. Memorise typical examples of each.
See the full worked example for transverse and longitudinal waves →
Two key examples.
Sound. Longitudinal pressure waves in matter. Requires a medium.
Speed in air at room temperature: ≈343 m/s. Increases with temperature; faster in solids and liquids than in gases.
Electromagnetic waves. Transverse. Electric and magnetic field oscillations perpendicular to each other AND to direction of travel.
Speed in vacuum: c=3.00×108 m/s. Does NOT need a medium.
Cambridge tip. EM waves don't need a medium — KEY distinction.
See the full worked example for transverse and longitudinal waves →
Longitudinal wave features.
Compression. Region where particles are BUNCHED together. High pressure and density.
Rarefaction. Region where particles are SPREAD apart. Low pressure and density.
Wavelength. Distance between adjacent compressions (or adjacent rarefactions).
Visualisation. Sound is often drawn as a sine wave plotting pressure vs position — but particle motion is along wave direction, not perpendicular.
Cambridge tip. When asked to sketch longitudinal wave, label particles and motion direction.
See the full worked example for transverse and longitudinal waves →
Verbatim phrases and definitions Cambridge mark schemes credit.
Wave types in every Paper 1. Most-tested: classification (5 marks), sketch & label (4-5 marks).
Sources: Cambridge International A Level Physics 9702 syllabus (2025-2027); 9702 Examiner Reports 2022-2024; 9702/22 May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-11.
Step-by-step solutions to past-paper-style questions on transverse and longitudinal waves, written exactly the way a tutor would explain them at the board.
Question
Classify each as transverse or longitudinal: (a) sound in air, (b) light, (c) ripples on water, (d) seismic P-waves, (e) seismic S-waves. (5 marks)
Step-by-step solution
Step 1
(a) Sound in air: LONGITUDINAL. Particles oscillate parallel to wave direction.
Step 2
(b) Light: TRANSVERSE. Electric and magnetic fields oscillate perpendicular.
Step 3
(c) Water ripples: TRANSVERSE (approximately).
Step 4
(d) P-waves: LONGITUDINAL. Primary, faster, travel through solid AND liquid.
Step 5
(e) S-waves: TRANSVERSE. Secondary, slower, only through solids.
Answer
(a) Long, (b) Trans, (c) Trans, (d) Long, (e) Trans.
Question
A 256 Hz tuning fork produces sound with wavelength 1.32 m. Find the speed of sound. (5 marks)
Step-by-step solution
Step 1
v=fλ.
v=256×1.32≈338 m/s
Answer
v≈338 m/s.
Question
Sketch a longitudinal wave. Label compressions and rarefactions. Describe particle motion. (4 marks)
Step-by-step solution
Step 1
Compression. Particles bunched together; high pressure.
Step 2
Rarefaction. Particles spread apart; low pressure.
Step 3
Particles oscillate parallel to wave direction. No net displacement.
Answer
Compression = bunched; rarefaction = spread. Parallel motion.
Definitions to memorise and the exact keywords mark schemes credit for transverse and longitudinal waves answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9702 sitting.
Particle oscillation PERPENDICULAR to direction of energy travel. Can be polarised. Examples: light, water ripples, S-waves.
Particle oscillation PARALLEL to direction of energy travel. Cannot be polarised. Compressions and rarefactions. Examples: sound, P-waves.
Region of high particle density and pressure in longitudinal wave.
Region of low particle density and pressure.
The traps other students keep falling into on transverse and longitudinal waves questions — taken from recent Cambridge International A Level 9702 examiner reports and mark schemes — and how to avoid them.
9702 Examiner Reports 2022-2024
Why it happens
Sound waves drawn as sine curves.
How to avoid it
Sound is LONGITUDINAL. The sine curve represents pressure vs position, not particle displacement perpendicular to motion.
9702 Examiner Reports 2022-2024
Why it happens
Sound needs air.
How to avoid it
EM waves (transverse) propagate through vacuum. Sound (longitudinal) requires medium.
The things students keep getting wrong in this sub-topic, answered.