Explain how longitudinal waves travel through a medium, referring to compressions and rarefactions. In your answer, also explain why the medium itself does not travel in the direction of the wave.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
In a longitudinal wave the particles of the medium vibrate parallel to the direction of wave propagation.
1. The vibration creates alternating regions of compression, where particles are pushed close together and pressure is high, and rarefaction, where particles are spread apart and pressure is low.
2. Each particle oscillates only about its equilibrium position; it does not move with the wave.
3. The disturbance is passed from one particle to the next, so the wave (energy) travels through the medium while the medium itself remains in place.
1. The vibration creates alternating regions of compression, where particles are pushed close together and pressure is high, and rarefaction, where particles are spread apart and pressure is low.
2. Each particle oscillates only about its equilibrium position; it does not move with the wave.
3. The disturbance is passed from one particle to the next, so the wave (energy) travels through the medium while the medium itself remains in place.
Examiner tips
- Use the term ‘compression’ and ‘rarefaction’ explicitly. Show that particles move parallel to the wave direction. Explain that only energy, not matter, is transferred. Keep the answer concise to fit the 5 marks.
Common mistakes
- Confusing particle motion with wave motion. Using ‘transverse’ instead of ‘longitudinal’. Forgetting to mention that particles oscillate about equilibrium.
Mark scheme (5 marks)
- In a longitudinal wave, the oscillations/vibrations of the particles are parallel to (in the same direction as) the direction of travel/energy transfer.
- Compressions are regions where particles are pushed closer together / regions of higher pressure.
- Rarefactions are regions where particles are spread further apart / regions of lower pressure.
- Each particle only oscillates/vibrates about its equilibrium (undisturbed) position / particles do not travel with the wave.
- The wave (energy) transfers through the medium by each particle passing its disturbance on to the next particle / it is energy that is transferred, not matter.
Key terms in this question
longitudinal wave · compression · rarefaction
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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