In a longitudinal wave, the oscillations of the medium are parallel to the direction the wave travels.
Features
- Regions where particles are pushed closer together are compressions.
- Regions where particles are spread further apart are rarefactions.
- The wavelength is the distance from one compression to the next (or one rarefaction to the next).
- Longitudinal waves cannot be polarised.
Examples of longitudinal waves
| Wave | Medium |
|---|---|
| Sound | Air, liquids, solids |
| P-waves (seismic) | Solids and liquids |
| Ultrasound | Air, liquids, solids |
| Pressure waves in a spring (slinky) | Spring |
How sound travels through air
- A vibrating source (speaker cone, tuning fork) pushes air molecules together, creating a compression.
- The molecules spread apart after the push, creating a rarefaction.
- The pattern of compressions and rarefactions travels through the air as a longitudinal wave.
- When it reaches the ear, the compressions and rarefactions cause the eardrum to vibrate.
Comparing transverse and longitudinal waves
| Feature | Transverse | Longitudinal |
|---|---|---|
| Vibration direction | Perpendicular to travel | Parallel to travel |
| Features | Crests and troughs | Compressions and rarefactions |
| Polarisation | Can be polarised | Cannot be polarised |
| Examples | Light, water waves | Sound, P-waves |
Common errors and how to correct them
Drawing a longitudinal wave as a transverse curve. Longitudinal waves should be drawn as a series of compressions (close-together dots or coils) and rarefactions (spread-apart dots or coils), not as an S-curve.
Saying sound is a transverse wave. Sound is always longitudinal. The air molecules vibrate back and forth parallel to the direction the sound travels.
How to apply this in an exam
State the definition: “vibrations are parallel to the direction of energy transfer.” Label compressions and rarefactions. If comparing with transverse, use a table with vibration direction, key features and examples.
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