What is the difference between transverse and longitudinal waves?
Transverse and longitudinal waves: the difference in the direction of oscillation, examples of each, and the structure of longitudinal waves as compressions and rarefactions.
A focused answer to Edexcel GCSE Physics 4.5, covering the difference between transverse and longitudinal waves, the direction of oscillation relative to energy transfer, examples including sound, electromagnetic, seismic and water waves, and the compressions and rarefactions in a longitudinal wave.
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What this dot point is asking
Edexcel statement 4.5 wants you to describe the difference between longitudinal and transverse waves by referring to sound, electromagnetic, seismic and water waves, including the direction of the oscillations relative to the direction of energy transfer.
Transverse waves
A water wave is the easy mental picture: the surface moves up and down while the wave travels horizontally across the pond. All electromagnetic waves (light, radio, microwaves and the rest) are transverse, and so are the secondary (S) seismic waves produced by earthquakes. You can model a transverse wave by shaking a rope up and down: the wave moves along the rope while each part of the rope moves only sideways.
Longitudinal waves
A longitudinal wave is well modelled by a stretched spring (a slinky) pushed and pulled along its length: a pulse of coils bunching together (a compression) travels along the spring, followed by a stretched-out region (a rarefaction). Because sound needs particles to compress and stretch, it cannot travel through a vacuum, unlike transverse electromagnetic waves.
Comparing the two
Both wave types obey the same wave-speed equation and share the same basic properties, so the distinction is purely about the direction in which the medium oscillates. Knowing which common waves are which (light transverse, sound longitudinal) is frequently tested.
How Edexcel examines this
This dot point appears on both tiers, usually as a description question contrasting the two wave types with an example of each, or as a structure question on sound using the terms compression and rarefaction. Mark schemes insist on the perpendicular-versus-parallel distinction relative to the direction of energy transfer, so do not just say "they vibrate differently"; state the direction precisely. The most common error, penalised every series, is classifying sound as transverse, so anchor sound firmly as longitudinal. For the sound-structure question, examiners reward identifying it as longitudinal and correctly defining compressions (particles close together, higher pressure) and rarefactions (particles spread apart, lower pressure). You may also be asked which seismic waves are which (P waves longitudinal, S waves transverse) and to use the fact that sound cannot travel through a vacuum because it needs particles, whereas transverse electromagnetic waves can. Diagrams of a slinky modelling each wave type are a frequent prompt.
Try this
Q1. State the direction of oscillation relative to energy transfer in a transverse wave. [1 mark]
- Cue. Perpendicular (at right angles) to the direction of energy transfer.
Q2. Name the regions of high and low pressure in a longitudinal sound wave. [2 marks]
- Cue. Compressions (high pressure) and rarefactions (low pressure).
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20203 marksDescribe the difference between a transverse wave and a longitudinal wave, and give one example of each.Show worked answer →
In a transverse wave the oscillations (vibrations) are at right angles (perpendicular) to the direction in which the wave transfers energy, for example an electromagnetic wave or a water wave (1 mark for the description, 1 mark for a transverse example). In a longitudinal wave the oscillations are parallel to (along) the direction of energy transfer, for example a sound wave (1 mark for the description and a longitudinal example). Markers reward the perpendicular-versus-parallel contrast and a valid example of each. Calling sound a transverse wave is the usual error.
Edexcel 20223 marksDescribe the structure of a sound wave travelling through air, using the terms compression and rarefaction.Show worked answer →
A sound wave is a longitudinal wave in which the air particles vibrate back and forth along the direction the wave travels (1 mark). This produces regions where the particles are pushed close together, called compressions (regions of higher pressure) (1 mark), and regions where they are spread apart, called rarefactions (regions of lower pressure) (1 mark). Markers reward identifying the wave as longitudinal and correctly defining compressions (particles close together) and rarefactions (particles spread out).
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Physics (1PH0) specification — Pearson (2016)