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Physics · Waves

Transverse and longitudinal waves

Describe the difference between transverse and longitudinal waves and describe evidence that it is the wave, and not the medium, that travels.

  • 6 key terms
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Warm-up

Answer each one, then check.

  1. 1

    What do waves transfer?

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    Energy

  2. 2

    Give an example of a wave.

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    Sound, light or water ripples

  3. 3

    What is a medium?

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    The material a wave travels through

  4. 4

    What is a vibration?

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    A repeated back-and-forth movement

  5. 5

    What do sound waves travel through?

    Show answerHide answer

    Solids, liquids and gases

Learning Objectives

  1. 1Describe the difference between transverse and longitudinal waves.
  2. 2Give examples of each.
  3. 3Describe compressions and rarefactions.
  4. 4Describe evidence that in ripples and sound it is the wave, not the water or air, that travels.

TWO TYPES OF WAVE

In a transverse wave the vibrations are at right angles to the direction the wave travels. In a longitudinal wave the vibrations are parallel to the direction of travel.

Waves transfer energy (and information) from one place to another without transferring matter.

Transverse or Longitudinal?

Transverse

  • Vibrations are at right angles to the direction of travel.
  • Examples: ripples on water, electromagnetic waves, waves on a rope.
  • Have crests and troughs.

Longitudinal

  • Vibrations are parallel to the direction of travel.
  • Examples: sound waves, P-waves.
  • Have compressions and rarefactions.

Evidence That the Wave Travels

Describe evidence that in water ripples it is the wave, and not the water, that moves.

Show the solutionHide the solution
  1. 1 Float a cork on a pond As ripples pass, the cork bobs up and down
  2. 2 Observe The cork does not travel along with the wave
  3. 3 Conclusion The water vibrates but does not move with the wave

AnswerThe cork moves up and down in the same place, so the water itself does not travel.

Sound in Air

Explain why sound waves are longitudinal.

Show the solutionHide the solution
  1. 1 Air particles Vibrate backwards and forwards in the direction the wave travels
  2. 2 Compressions Regions where the particles are pushed close together
  3. 3 Rarefactions Regions where the particles are spread out

AnswerSound is a longitudinal wave with compressions and rarefactions.

Key Points

Learn these.

  • Energy, not matter

    Waves transfer energy without transferring matter.

  • Medium

    Mechanical waves need a medium; electromagnetic waves do not.

  • Sound evidence

    Particles in air vibrate about fixed positions; smoke in a tube just vibrates back and forth.

  • Don't say

    'The water moves along'. The water just moves up and down.

Wave Types

Classify each wave as transverse or longitudinal: (a) light (b) sound in air (c) ripples on a pond (d) a wave on a rope shaken up and down.

1. Is vibration at right angles?

2. Or parallel?

A good answer shows: (a) Transverse (b) Longitudinal (c) Transverse (d) Transverse.

Can I...?

  1. 1Define transverse.
  2. 2Define longitudinal.
  3. 3Give examples.
  4. 4Name compressions and rarefactions.
  5. 5Describe the cork evidence.
  6. 6Say waves transfer energy.
  7. 7Explain sound in air.
  8. 8Draw both wave types.

Summary & Exam Focus

  • Transverse: vibrations at right angles; longitudinal: parallel.
  • Sound is longitudinal; ripples are transverse.
  • Compressions and rarefactions in longitudinal waves.
  • The wave, not the medium, travels.

Exam focus

Describe the difference between a transverse wave and a longitudinal wave. (2 marks) (2 marks)

Direction of vibration relative to the direction of travel.

Key terms

The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.

Transverse wave
A wave with vibrations at right angles to the direction of travel.
Longitudinal wave
A wave with vibrations parallel to the direction of travel.
Compression
A region where particles are close together.
Rarefaction
A region where particles are spread out.
Medium
The material a wave travels through.
Oscillation
A back-and-forth movement.

Practice questions

Have a go at each one before you open its answer.

  1. Question 1 Describe 2 marks

    Describe the difference between a transverse wave and a longitudinal wave.

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    Model answer

    In a transverse wave the vibrations are at right angles to the direction of travel of the wave. In a longitudinal wave the vibrations are parallel to the direction of travel.

    Mark scheme

    • Transverse: at right angles — 1 mark
    • Longitudinal: parallel — 1 mark
  2. Question 2 State 2 marks

    Give one example of a transverse wave and one example of a longitudinal wave.

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    Model answer

    Transverse: ripples on water, light or other electromagnetic waves. Longitudinal: sound waves in air.

    Mark scheme

    • A transverse wave — 1 mark
    • A longitudinal wave — 1 mark
  3. Question 3 Describe 3 marks

    Describe how a longitudinal wave, such as a sound wave, travels through air. Use the words compression and rarefaction.

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    Model answer

    The air particles vibrate back and forth parallel to the direction the wave travels. The wave consists of regions where the particles are pushed together (compressions) and regions where they are spread out (rarefactions), which move through the air.

    Mark scheme

    • Particles vibrate parallel to travel — 1 mark
    • Compressions: particles close together — 1 mark
    • Rarefactions: particles spread out — 1 mark
  4. Question 4 Describe 3 marks

    A cork floats on water. Ripples pass it. Describe what happens to the cork and explain what this shows about the wave.

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    Model answer

    The cork moves up and down but does not travel along with the wave. This shows that the wave transfers energy but the water itself does not move along.

    Mark scheme

    • Cork moves up and down — 1 mark
    • Cork does not move along with the wave — 1 mark
    • The wave, not the water, travels (energy transferred) — 1 mark
  5. Question 5 Explain 3 marks

    Explain why sound cannot travel through a vacuum but light can.

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    Model answer

    Sound is a longitudinal wave that needs particles to vibrate, so it needs a medium. Light is an electromagnetic wave and does not need a medium.

    Mark scheme

    • Sound needs a medium or particles — 1 mark
    • No particles in a vacuum — 1 mark
    • Light does not need a medium — 1 mark

Quick check

  1. Sound waves are...

    1. Atransverse
    2. Blongitudinal
    3. Celectromagnetic
    4. Dstatic
    Show answerHide answer

    B: longitudinal

    Compressions and rarefactions.

  2. In a transverse wave vibrations are...

    1. Aparallel to the direction
    2. Bzero
    3. Cat right angles to the direction of travel
    4. Drandom
    Show answerHide answer

    C: at right angles to the direction of travel

    That defines it.

  3. A compression is a region where particles are...

    1. Afar apart
    2. Bstationary
    3. Chot
    4. Dclose together
    Show answerHide answer

    D: close together

    Higher pressure.

  4. Waves transfer...

    1. Aenergy
    2. Bmatter only
    3. Cmass
    4. Dcharge only
    Show answerHide answer

    A: energy

    Not matter.

  5. A cork on water bobs up and down. This shows the...

    1. Acork is magnetic
    2. Bwater does not travel with the wave
    3. Cwave is longitudinal
    4. Dwater is solid
    Show answerHide answer

    B: water does not travel with the wave

    Only the wave travels.

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