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This lesson introduces the two fundamental types of wave — transverse and longitudinal — as required by the AQA GCSE Combined Science Trilogy specification (8464), Physics Paper 2, section 6.1. Waves transfer energy from one place to another without transferring matter. A secure understanding of these wave types underpins every other topic in the Waves chapter.
A wave is a disturbance that transfers energy from one place to another. The key principle for your exam 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 (AQA 8464): 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["Energy transfer →"] --- B["↑ Crest"]
B --- C["↓ Trough"]
C --- D["↑ Crest"]
D --- E["↓ Trough"]
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° 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. Draw a double-headed arrow for oscillation direction and a single arrow for the direction of energy transfer in your diagram.
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["|||| Compression"] --- B[" | | | Rarefaction"]
B --- C["|||| Compression"]
C --- D[" | | | Rarefaction"]
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:
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 (crests) 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 both types of wave:
| Demonstration | Wave type | What to observe |
|---|---|---|
| Shaking a rope / slinky sideways | Transverse | Crests and troughs travel along the rope while each point moves up and down |
| Pushing and pulling a slinky along its length | Longitudinal | Compressions and rarefactions travel along the spring while each coil moves back and forth |
| Ripple tank with a straight dipper | Transverse (surface water waves) | Wavefronts travel outward; water particles move up and down |
| Mistake | Correction |
|---|---|
| "Waves transfer particles from place to place" | Waves transfer energy, not particles |
| "Sound is a transverse wave" | Sound is a longitudinal wave |
| "All waves need a medium" | EM waves can travel through a vacuum |
| "Wavelength in longitudinal waves is crest to crest" | Use compression to compression for longitudinal waves |
| Confusing perpendicular and parallel | Transverse = perpendicular; Longitudinal = parallel |