Explain why sound waves are described as longitudinal waves and how they are able to travel through air.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
In a longitudinal wave the particles of the medium vibrate parallel to the direction of wave travel.
The vibrating source pushes air particles together, creating compressions, and pulls them apart, creating rarefactions.
These alternating compressions and rarefactions propagate through the air, transferring energy from one particle to the next.
Sound therefore requires a material medium – it cannot travel through a vacuum.
The vibrating source pushes air particles together, creating compressions, and pulls them apart, creating rarefactions.
These alternating compressions and rarefactions propagate through the air, transferring energy from one particle to the next.
Sound therefore requires a material medium – it cannot travel through a vacuum.
Examiner tips
- Use the definition of a longitudinal wave – particles move parallel to wave direction. Show the source creates compressions and rarefactions. Explain that these disturbances travel through the medium. Mention that a medium is required – no propagation in vacuum.
Common mistakes
- Confusing transverse with longitudinal motion. Forgetting to mention that sound needs a medium. Using vague terms like "push and pull" without linking to compressions/rarefactions.
Mark scheme (4 marks)
- In a longitudinal wave, the particles vibrate/oscillate parallel to the direction of energy transfer (or direction of wave travel)
- The vibrating source causes air particles to be pushed together, forming compressions, and pulled apart, forming rarefactions
- These compressions and rarefactions travel through the air, transferring energy
- Sound requires a medium (a material/particles) through which to travel and cannot travel through a vacuum
Key terms in this question
Related
- All Eduqas GCSE Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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