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This lesson introduces the two fundamental types of wave — transverse and longitudinal — as required by the AQA GCSE Physics specification (4.6.1). Waves transfer energy from one place to another without transferring matter. Understanding the difference between these two wave types is essential for every topic in the Waves chapter and appears frequently in exam questions.
A wave is a disturbance that transfers energy from one place to another. The key point is that waves transfer energy, not matter. The particles of the medium oscillate (vibrate) about their rest position but do not travel along with the wave.
Key facts about waves:
Exam Tip: A very common exam question asks "What do waves transfer?" The answer is always energy. Never say waves transfer "matter" or "particles." The particles vibrate but stay in roughly the same position.
In a transverse wave, the oscillations (vibrations) of the particles are perpendicular (at right angles) to the direction of energy transfer.
graph LR
subgraph "Transverse Wave"
direction LR
A["Direction of energy transfer -->"]
end
Imagine shaking a rope up and down: the wave travels horizontally along the rope, but the particles of the rope move up and down (vertically). The oscillations are at 90 degrees to the direction the wave moves.
| Wave Type | Medium | Notes |
|---|---|---|
| Light (all EM waves) | Can travel through a vacuum | Part of the electromagnetic spectrum |
| Water waves (surface) | Water surface | Particles move up and down |
| S-waves (seismic) | Solid rock only | Cannot travel through liquids |
| Waves on a string or rope | String / rope | Classic classroom demonstration |
Exam Tip: If asked to identify whether a wave is transverse, look for the keyword perpendicular. The oscillations must be at right angles to the direction the wave travels. Always draw a double-headed arrow for oscillation direction and a single arrow for the direction of energy transfer.
In a longitudinal wave, the oscillations of the particles are parallel to the direction of energy transfer. The particles vibrate back and forth in the same direction the wave is moving.
graph LR
subgraph "Longitudinal Wave"
direction LR
A["Direction of energy transfer -->"]
B["<-- Particle oscillation -->"]
end
Imagine pushing and pulling a slinky spring horizontally: the coils bunch together (compressions) and spread apart (rarefactions) as the wave travels along.
| Wave Type | Medium | Notes |
|---|---|---|
| Sound waves | Solids, liquids, gases | Cannot travel through a vacuum |
| Ultrasound | Solids, liquids, gases | Frequency above 20 000 Hz |
| P-waves (seismic) | Solids and liquids | Faster than S-waves |
| Waves in a slinky spring | Spring | Compressions and rarefactions visible |
Longitudinal waves consist of alternating regions of:
graph LR
C1["|||"] --- R1[" | | | "] --- C2["|||"] --- R2[" | | | "] --- C3["|||"]
style C1 fill:#3498db,color:#fff
style C2 fill:#3498db,color:#fff
style C3 fill:#3498db,color:#fff
style R1 fill:#ecf0f1,color:#333
style R2 fill:#ecf0f1,color:#333
Exam Tip: In longitudinal waves, the wavelength is measured from the centre of one compression to the centre of the next compression (or from one rarefaction to the next). Do not confuse compressions with peaks — peaks and troughs only apply to transverse waves.
| Feature | Transverse Wave | Longitudinal Wave |
|---|---|---|
| Oscillation direction | Perpendicular to energy transfer | Parallel to energy transfer |
| Examples | Light, water waves, S-waves | Sound, ultrasound, P-waves |
| Features | Peaks and troughs | Compressions and rarefactions |
| Can be polarised? | Yes | No |
| Can travel through a vacuum? | Some (EM waves) | No |
| Need a medium? | EM waves do not; others do | Always need a medium |
Waves can also be classified as mechanical or electromagnetic:
graph TD
W["Waves"] --> M["Mechanical Waves"]
W --> E["Electromagnetic Waves"]
M --> MT["Can be transverse or longitudinal"]
M --> MR["Require a medium"]
E --> ET["Always transverse"]
E --> ER["Can travel through a vacuum"]
style W fill:#2c3e50,color:#fff
style M fill:#2980b9,color:#fff
style E fill:#e74c3c,color:#fff
style MT fill:#3498db,color:#fff
style MR fill:#3498db,color:#fff
style ET fill:#c0392b,color:#fff
style ER fill:#c0392b,color:#fff
In the classroom, you can demonstrate each type of wave using a slinky spring:
A displacement-distance graph can represent both transverse and longitudinal waves.
| Graph Feature | Transverse Wave | Longitudinal Wave |
|---|---|---|
| Above the axis | Peak (crest) | Particle displaced in the direction of wave travel |
| Below the axis | Trough | Particle displaced opposite to wave travel |
| At the axis (zero displacement) | Particle at rest position | Centre of compression or rarefaction |
Exam Tip: A common 4-mark question asks you to "compare transverse and longitudinal waves." For full marks, state the direction of oscillation relative to the direction of energy transfer for both types, name an example of each, and mention that transverse waves can be polarised but longitudinal waves cannot.
The wave equation v = f × lambda applies to both transverse and longitudinal waves, because they both have a well-defined wavelength, frequency and wave speed. The only differences lie in how the wavelength is measured on a diagram (peak-to-peak for transverse; compression-to-compression for longitudinal).
Question: A sound wave (longitudinal) of frequency 170 Hz travels through air at 340 m/s. Calculate its wavelength. Then calculate the wavelength of a transverse radio wave of the same frequency travelling through a vacuum at 3 × 10⁸ m/s.
Step 1 — sound wave:
lambda = v / f = 340 / 170 = 2.0 m
Step 2 — radio wave:
lambda = v / f = (3 × 10⁸) / 170 = 1.76 × 10⁶ m (about 1 760 km)
This shows why, although the same equation applies, the scale of a longitudinal sound wave and a transverse radio wave is completely different — the radio wavelength is roughly a million times longer even though their frequencies are identical.
Common mistake: Students often write "sound is a transverse wave" because they picture sound drawn as a wavy line on a displacement–distance graph. The graph is a representation of particle displacement, not the actual motion of the air particles. The air particles themselves oscillate back and forth parallel to the wave, so sound is longitudinal.
Because the oscillations of a transverse wave can be in any plane perpendicular to the direction of travel, they can be filtered so that only one plane of oscillation remains. This is called polarisation and is only possible for transverse waves.
Common mistake: Students sometimes write that "longitudinal waves can be polarised in a spring." They cannot. Only transverse waves can be polarised, and this is a favourite discriminating question at higher grades.
Question: "Describe the difference between a transverse and a longitudinal wave. Give one example of each and explain one property that only applies to transverse waves." (5 marks)
Grade 4–5 answer:
In a transverse wave the particles move up and down. In a longitudinal wave the particles move back and forth. An example of a transverse wave is light and an example of a longitudinal wave is sound. Only transverse waves can be polarised.
This scores the basic marks for identifying the two types and giving examples, but it does not use the precise language examiners look for and does not explain polarisation.
Grade 8–9 answer:
In a transverse wave the oscillations of the particles (or field) are perpendicular to the direction of energy transfer — for example light, where the electric and magnetic fields oscillate at right angles to the direction the wave travels. In a longitudinal wave the oscillations are parallel to the direction of energy transfer, producing regions of high pressure (compressions) and low pressure (rarefactions); sound in air is a clear example. Only transverse waves can be polarised because the plane of oscillation can be restricted by a polarising filter. Longitudinal waves cannot be polarised because their oscillations are already confined to the single direction along which the wave travels. Both wave types still transfer energy without transferring matter and both obey v = f × lambda.
The higher-level answer uses the exact specification language (perpendicular/parallel, compression/rarefaction), gives a correct mechanism for polarisation, links to the wave equation, and addresses the common misconception that longitudinal waves could be polarised.
AQA alignment: This content is aligned with AQA GCSE Physics (8463) specification section 4.6 Waves — specifically 4.6.1.1 Transverse and longitudinal waves and 4.6.1.2 Properties of waves. Assessed on Paper 2.