Longitudinal and transverse waves — AQA A-Level Physics

This topic introduces the fundamental classification of waves into longitudinal and transverse types, based on the direction of particle oscillation relative to the direction of energy transfer. You will learn the key definitions, examples, and graphical representations of each wave type, and how to apply the wave equation v = f lambda to solve problems.

Understanding wave types is essential for later topics such as superposition, interference, stationary waves, and polarisation. It also underpins the study of electromagnetic waves, sound, and seismic waves. AQA A-Level exams frequently test your ability to describe, compare, and explain the behaviour of longitudinal and transverse waves in both theoretical and practical contexts.

• →Transverse waves: oscillations are perpendicular to the direction of energy transfer; examples include electromagnetic waves, waves on a string, and S-waves.
• →Longitudinal waves: oscillations are parallel to the direction of energy transfer; they consist of compressions and rarefactions; examples include sound waves and P-waves.
• →The wave equation v = f lambda relates wave speed, frequency, and wavelength; it applies to both wave types.
• →Polarisation is the restriction of oscillations to a single plane and is only possible for transverse waves; it provides evidence for the transverse nature of light.
• →Graphical representations: transverse waves show peaks and troughs; longitudinal waves show compressions and rarefactions, often represented as displacement-position graphs.

• States that transverse waves have particle oscillation perpendicular to the direction of energy transfer.
• States that longitudinal waves have particle oscillation parallel to the direction of energy transfer.
• Identifies compressions and rarefactions as the regions of high and low particle density in a longitudinal wave.
• Classifies given examples correctly, such as sound as longitudinal and light or a wave on a string as transverse.
• Links transverse nature to polarisation and explains that longitudinal waves cannot be polarised.
• Sound is longitudinal because the oscillations of the particles are parallel to the direction of energy transfer.
• Electromagnetic waves are transverse because the electric and magnetic field oscillations are perpendicular to the direction of energy transfer.
• Waves on a string are transverse because the displacement of string elements is perpendicular to the direction of energy transfer along the string.
• A valid classification must name the oscillating quantity and compare its direction with the direction of energy propagation.
• Electromagnetic waves are the example that does not require a material medium, unlike sound and waves on a string.
• Longitudinal waves have particle or field displacement parallel to the direction of energy propagation.
• Transverse waves have particle or field displacement perpendicular to the direction of energy propagation.
• Classification requires both directions to be stated and compared, not just one of them.
• All electromagnetic waves travel at the same speed in a vacuum, c = 3.00 × 10⁸ m s⁻¹.
• The vacuum speed is independent of frequency and wavelength, so it is a shared property of the whole electromagnetic spectrum.
• Polarisation restricts oscillations to a single plane containing the direction of propagation.
• Unpolarised transverse waves oscillate in all planes perpendicular to the direction of travel.
• A polarising filter transmits only oscillations parallel to its transmission axis, so intensity falls.
• Longitudinal waves oscillate parallel to the direction of travel and therefore cannot be polarised.
• Observation of polarisation is direct evidence that the wave is transverse, because only transverse waves have perpendicular oscillation directions that can be selected.
• Polaroid transmits oscillations parallel to its transmission axis and absorbs or blocks other planes.
• Polaroid sunglasses reduce glare because light reflected from horizontal surfaces is partly horizontally polarised.
• A transmitting aerial emits an electromagnetic wave with electric field oscillations along the aerial's alignment.
• A receiving aerial detects the signal best when aligned parallel to the incoming electric field oscillations.
• Rotating the receiving aerial towards perpendicular alignment reduces the received signal, demonstrating polarisation selection.

• 💡Use the words perpendicular and parallel explicitly when comparing the two wave types.
• 💡When classifying an example, state the direction of particle oscillation relative to energy transfer, not just the name of the wave.
• 💡For longitudinal waves, label compressions and rarefactions on any diagram to show understanding of particle spacing.
• 💡For each example, state the oscillating quantity and then compare its direction with the direction of energy transfer.
• 💡Use the words parallel and perpendicular explicitly; vague phrases such as 'side to side' do not show the required comparison.
• 💡When asked for examples, give the three named in the specification and add the classification for each rather than listing them alone.
• 💡Write the comparison as a full sentence naming both directions, for example displacement is perpendicular to energy propagation.
• 💡Quote the vacuum speed with its value and unit, and state that it applies to all electromagnetic waves.
• 💡If a question gives frequency and wavelength, check whether their product equals the vacuum speed before using the data.
• 💡State explicitly that the oscillations are confined to one plane and link this to the transverse nature of the wave.
• 💡Use the polariser pair test: rotating the analyser changes transmitted intensity, reaching a minimum when axes are perpendicular.
• 💡Contrast with a longitudinal wave such as sound to show why polarisation is impossible for it.
• 💡Link the transmission axis direction to the oscillation direction that passes through.
• 💡For aerials, state that the receiving aerial must be parallel to the electric field oscillations for maximum reception.
• 💡Use a clear application such as glare reduction or radio reception to show understanding rather than just naming Polaroid.
• 💡When defining wave types, always refer to the direction of oscillation relative to the direction of energy transfer, using the terms 'perpendicular' and 'parallel'.
• 💡For polarisation questions, explicitly state that it is the restriction of oscillations to a single plane and that this is only possible for transverse waves.
• 💡In calculations, always write the wave equation, substitute values with units, and give your final answer with the correct unit and significant figures.

• Thinking that all waves in solids are longitudinal; the correction is that both transverse and longitudinal waves can travel through solids, and classification depends on oscillation direction.
• Believing that sound can travel through a vacuum; the correction is that sound is a mechanical longitudinal wave and needs a medium.
• Assuming that a wave shown as a sine curve must be transverse; the correction is that a displacement–distance graph can represent a longitudinal wave even though the particles oscillate along the direction of travel.
• Calling sound transverse because it can be reflected or refracted; correction: classify by the direction of particle oscillation relative to energy transfer, so sound in a fluid is longitudinal.
• Saying electromagnetic waves are longitudinal because they can travel through a vacuum; correction: the absence of a medium does not determine wave type, and electromagnetic waves are transverse.
• Describing a wave on a string as longitudinal because the string is stretched along its length; correction: the tension is along the string, but the displacement of each element is perpendicular to the energy transfer.
• Treating the direction of energy propagation as the same as the direction of particle displacement for every wave; correction: compare the two directions and classify as parallel or perpendicular.
• Believing that higher-frequency electromagnetic waves travel faster in a vacuum; correction: all electromagnetic waves have the same vacuum speed, and frequency changes only the wavelength.
• Assuming electromagnetic waves slow down in a vacuum as they do in glass or water; correction: the reduced speed occurs in a material medium, not in a vacuum.
• Saying polarisation removes half the oscillations in every direction; the correction is that it selects one plane and reduces intensity according to the angle between axes.
• Claiming sound can be polarised because it is a wave; the correction is that sound is longitudinal, so its oscillations are along the direction of travel and no perpendicular plane exists to select.
• Confusing polarisation with reflection or diffraction; the correction is that polarisation is a property of transverse oscillations, not a change in direction or spreading.
• Thinking Polaroid blocks all light; the correction is that it transmits the component parallel to its axis, so intensity is reduced rather than necessarily zero.
• Believing aerial orientation does not matter for reception; the correction is that a receiving aerial aligned perpendicular to the electric field oscillations picks up little or no signal.
• Assuming Polaroid sunglasses work by darkening all light equally; the correction is that they selectively reduce polarised glare according to the transmission axis orientation.
• Students often think that longitudinal waves cannot be represented graphically or that they do not have wavelength or frequency. In fact, they can be represented as displacement-position graphs, and the same wave equation applies.
• Many believe that polarisation can occur for any wave. Polarisation is exclusive to transverse waves because it requires oscillations perpendicular to the direction of energy transfer.
• Some students confuse the direction of particle oscillation with the direction of wave travel. Always specify that oscillation direction is relative to energy transfer, not just 'up/down' or 'left/right'.

1. 1Day 1: Learn the definitions of longitudinal and transverse waves, including examples and graphical representations. Use flashcards to memorise key terms.
2. 2Day 2: Practice applying the wave equation to both wave types. Solve problems involving frequency, wavelength, and speed.
3. 3Day 3: Study polarisation: definition, conditions, and evidence for transverse nature of light. Complete past-paper questions on polarisation.
4. 4Day 4: Compare and contrast longitudinal and transverse waves in a table, including oscillation direction, examples, polarisation, and graphical representation.
5. 5Day 5-7: Attempt exam-style questions, focusing on 4-6 mark explanations. Review mark schemes and refine your answers to include precise terminology.

• 📋Definition and comparison questions: 'State two differences between longitudinal and transverse waves.' Use precise terms: parallel/perpendicular, compressions/rarefactions, polarisation.
• 📋Calculation questions: 'Calculate the speed of a wave given frequency and wavelength.' Show the equation, substitution, and units.
• 📋Explanation questions: 'Explain why sound waves cannot be polarised.' Link polarisation to transverse waves and oscillation direction.
• 📋Practical application questions: 'Describe an experiment to demonstrate polarisation of microwaves.' Outline the use of a metal grille and receiver, and explain the effect of rotating the grille.