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3.1.1Waves

Transverse and Longitudinal Waves — Edexcel International GCSE Science Double Award 4SD0 - Physics Revision Notes & Topical Questions

5 min read 13 parts Lesson 2 of 20

📚 Edexcel International GCSE Science Double Award 4SD0 - Physics Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
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Part 1 of 7What a wave does

A wave is a regular disturbance that carries energy from one place to another without carrying matter with it.

What a wave does
The particles of a medium, such as water or air, move to and fro about a fixed position while the energy travels outward.
A floating cork on a pond bobs up and down as ripples pass but does not travel with the ripples.
Some waves, such as light, need no medium at all and can travel through a vacuum.

Part 2 of 7Transverse waves

In a transverse wave the oscillations are at right angles to the direction in which the wave transfers energy. The wave has high points called crests (peaks) and low points called troughs.

1
Water ripples (the surface moves up and down while the wave moves across).
2
Waves on a stretched rope or spring when you shake one end sideways.
3
All electromagnetic waves: light, radio, X-rays and so on. These are oscillations of electric and magnetic fields, not of particles.
4
S-waves from earthquakes.

Part 3 of 7Longitudinal waves

In a longitudinal wave the oscillations are parallel to the direction of energy transfer.

Compressions
The wave consists of regions where the particles are pushed together, called compressions,
  • and regions where they are spread apart, called rarefactions.
Sound is the key example, along with P-waves from earthquakes and the pulses along a slinky spring when it is pushed and pulled along its length.

In a compression the pressure is above average; in a rarefaction it is below average. Longitudinal waves need a medium: sound cannot travel through a vacuum, since there are no particles to be compressed.

Part 4 of 7Comparing the two types

FeatureTransverseLongitudinal
Oscillation directionPerpendicular to travelParallel to travel
Parts of the waveCrests and troughsCompressions and rarefactions
Needs a medium?Not always (EM waves do not)Always
ExamplesLight, water ripples, rope wavesSound, ultrasound, slinky pushes

Both types can be reflected, refracted and diffracted. Only transverse waves can be polarised, which is why light is shown to be transverse.

Part 5 of 7Seeing the wave in a drawing

A transverse wave is drawn as a displacement–distance graph.

1
The horizontal axis is the position along the wave and the vertical axis is the displacement of the particles from rest.
2
A longitudinal wave is often drawn as a line of dots, close together in compressions and spread out in rarefactions.
3
If the compressions and rarefactions are plotted as a graph of displacement against distance, a longitudinal wave also looks like a sine curve.
4
Details such as wavelength and amplitude are covered in Describing Wave Motion, and the wave speed in The Wave Equation.

Part 6 of 7Demonstrating the two types with a spring

A long coiled spring (a slinky) is the classic way to see both types.

Demonstrating the two types with a spring
Fix one end and move the other end sideways: a transverse pulse travels along the spring, and each coil moves from side to side as it passes.
Now push the end sharply along the length of the spring and pull it back: bunched-up coils (a compression) travel along, followed by a stretched region (a rarefaction),
  • and each coil moves forwards and backwards along the line of travel.
In both cases a coil returns to where it started once the pulse has passed, which shows that only energy moves along the spring.

In seismology the two types are both important.

Demonstrating the two types with a spring
P-waves are longitudinal and travel through solids and liquids; S-waves are transverse and travel only through solids.
Since S-waves are not detected on the far side of the Earth, scientists deduced that the outer core is liquid.
This link between wave type and the medium is a good example of how wave properties are used to find out about things that cannot be seen directly.

Ripples on water are often called transverse, but the water particles actually move in small circles, so the wave is a mixture; at this level the simple description of a transverse wave on the surface is accepted.

Part 7 of 7Worked examples

  1. A student shakes the end of a long spring up and down and sees pulses travelling along it. Say whether the wave is transverse or longitudinal, and what she must do to make a longitudinal wave.

  2. Explain why astronauts cannot hear each other through the vacuum of space, yet can see each other.

Key terms

TermMeaning
WaveDisturbance that transfers energy without transferring matter
Transverse waveOscillations at right angles to the direction of energy transfer
Longitudinal waveOscillations parallel to the direction of energy transfer
CompressionRegion of a longitudinal wave where the particles are closest together
RarefactionRegion where particles are furthest apart
MediumMaterial through which a wave travels

Avoid these mistakes

1
Saying the particles travel along with the wave – they oscillate about a fixed point.
2
Confusing compressions (high pressure) with crests.
3
Describing sound as transverse.
4
Thinking all waves need a medium – EM waves do not.
5
Drawing the sound wave as a sine wave and assuming the particles move up and down.

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