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Waves are one of the most fundamental concepts in physics — they transfer energy from one place to another without transferring matter. This lesson introduces the two types of wave (transverse and longitudinal), their key features, and how to identify and compare them, as required by AQA GCSE Combined Science Trilogy (8464), Physics Paper 2, section 6.1.
A wave is a disturbance that transfers energy from one place to another. The crucial principle: waves transfer energy, not matter. The particles of the medium vibrate about their equilibrium position but do not travel with the wave.
Key facts:
Exam Tip (AQA 8464): The single most common exam question on this topic: "What do waves transfer?" The answer is always energy. Never write "matter" or "particles."
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 each part of the rope moves up and down (vertically). The oscillations are at 90° to the wave's direction of travel.
| Wave Type | Medium | Notes |
|---|---|---|
| Light (all EM waves) | Can travel through a vacuum | Part of the electromagnetic spectrum |
| Water waves (surface) | Water surface | Particles actually move in circular paths; surface motion is approximately transverse |
| S-waves (seismic) | Solid rock only | Cannot travel through liquids — used to deduce Earth's liquid outer core |
| Waves on a string or rope | String / rope | Classic classroom demonstration |
In a longitudinal wave, the oscillations of the particles are parallel to the direction of energy transfer. The particles vibrate back and forth along the same direction the wave travels.
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 (above human hearing range) |
| P-waves (seismic) | Solids and liquids | Faster than S-waves; first to arrive after an earthquake |
| Waves in a slinky spring (push-pull) | Spring | Compressions and rarefactions are visible |
Exam Tip: The wavelength of a longitudinal wave is measured from the centre of one compression to the centre of the next compression (or one rarefaction to the next). Do not confuse compressions with peaks — peaks and troughs apply to transverse waves only.
| Feature | Transverse | Longitudinal |
|---|---|---|
| Direction of oscillation | Perpendicular to energy transfer | Parallel to energy transfer |
| Key features | Crests and troughs | Compressions and rarefactions |
| Can be polarised? | Yes | No |
| Can travel through a vacuum? | Only EM waves (not mechanical transverse waves like rope waves) | No (longitudinal waves need a medium) |
| Examples | Light, water waves, S-waves | Sound, ultrasound, P-waves |
graph TD
A["Wave Type?"] --> B{"Oscillation direction\nvs energy transfer"}
B -->|"Perpendicular (90°)"| C["TRANSVERSE"]
B -->|"Parallel (same direction)"| D["LONGITUDINAL"]
C --> E["Examples: light, EM waves,\nwater surface, S-waves"]
D --> F["Examples: sound,\nultrasound, P-waves"]
C --> G["Can be polarised ✓"]
D --> H["Cannot be polarised ✗"]
| Type | Needs a Medium? | Examples |
|---|---|---|
| Mechanical | Yes — must have particles to vibrate | Sound, water waves, seismic waves, waves on a slinky |
| Electromagnetic | No — can travel through a vacuum | Light, radio, microwaves, X-rays, etc. |
Sound is a mechanical, longitudinal wave. Light is an electromagnetic, transverse wave.
A wave travels through a spring. The coils vibrate back and forth along the direction the wave travels. Is this a transverse or longitudinal wave? Justify your answer.
Answer: This is a longitudinal wave because the oscillations of the coils are parallel to the direction of energy transfer (both along the length of the spring). Longitudinal waves in a spring produce compressions and rarefactions.
Explain why sound cannot travel through a vacuum but light can.
Answer: Sound is a mechanical, longitudinal wave — it requires particles to vibrate (compressions and rarefactions). A vacuum contains no particles, so there is nothing to compress or rarefy, and the wave cannot be transmitted. Light is an electromagnetic wave — it does not need particles. EM waves are oscillations of electric and magnetic fields, which can propagate through empty space.
Explain why seismic S-waves cannot travel through the Earth's liquid outer core.
Answer: S-waves are transverse waves — the particles vibrate perpendicular to the direction of travel. For this to work, the particles must be able to exert sideways (shear) forces on each other. In a solid, the particles are held in fixed positions by strong bonds, so they can transmit shear forces. In a liquid, the particles are free to flow past each other and cannot support shear forces, so the transverse wave cannot propagate. This is why the shadow zone for S-waves provides evidence that the Earth's outer core is liquid.
| Mistake | Correction |
|---|---|
| "Waves transfer matter" | Waves transfer energy, not matter |
| "Sound is a transverse wave" | Sound is a longitudinal wave |
| "Longitudinal waves have crests and troughs" | Longitudinal waves have compressions and rarefactions |
| "All waves need a medium" | Electromagnetic waves can travel through a vacuum |
| Confusing perpendicular and parallel | Transverse = perpendicular oscillation; longitudinal = parallel oscillation |
| Key Concept | Transverse | Longitudinal |
|---|---|---|
| Oscillation direction | Perpendicular to energy transfer | Parallel to energy transfer |
| Features | Crests, troughs | Compressions, rarefactions |
| Polarisation | Possible | Not possible |
| Examples | Light, water surface waves, S-waves | Sound, P-waves, ultrasound |