IB Physics SLTopic C.2 — Modelling WavesPaper 1 & 2Transverse vs longitudinal~7 min read
Transverse & Longitudinal Waves
All waves shift energy along, but they don’t all wiggle the same way. The single question that sorts them into two families is: does the material shake across the direction of travel, or along it?
📘 What you need to know
In a wave the particles just oscillate about fixed points — they don’t travel along with the wave
The two types are set by the oscillation direction relative to the direction of travel
Transverse: particles oscillate perpendicular to the travel direction — think up and down
Longitudinal: particles oscillate parallel to the travel direction — think back and forth
Transverse waves have crests and troughs; longitudinal waves have compressions and rarefactions
All EM waves are transverse (and can cross a vacuum); sound is longitudinal (and needs a medium)
The Deciding Question
Whatever the wave, the particles of the medium stay roughly where they are — they wobble about a fixed spot and pass the energy on to their neighbours. The illusion of movement along the wave comes from all those particles wobbling slightly out of step. What separates the two wave types is simply which way that wobble points compared with the direction the energy is heading.
Transverse Waves
In a transverse wave, the oscillations are perpendicular to the direction of motion and energy transfer — each particle bobs up and down while the wave rolls sideways. These are the wavy-line waves, with high points called crests (peaks) and low points called troughs.
The wave travels sideways (blue) while each particle only moves up and down (red) — perpendicular motion, with crests and troughs.
Crucially, transverse waves transfer energy without needing particles to pass it on — so they can travel through a vacuum. That’s why sunlight and its UV reach us across the empty vacuum of space. Examples include all electromagnetic waves (radio, visible light, UV) and mechanical ones like the vibrations on a plucked guitar string.
Longitudinal Waves
In a longitudinal wave, the oscillations are parallel to the direction of motion — each particle shuffles back and forth along the same line the wave travels. Instead of crests and troughs you get regions where particles bunch up and regions where they spread out:
A compression is a region of high pressure, where particles are squeezed close together
A rarefaction is a region of low pressure, where particles are stretched further apart
Vertical lines are the particles. Where they crowd together is a compression (high pressure); where they spread out is a rarefaction (low pressure). Travel (blue) and oscillation (red) point the same way.
Longitudinal waves are mechanical — they need particles to carry the compressions along, so they cannot travel through a vacuum. That’s why space is silent: with no air, a sound wave has nothing to push. Examples are sound and ultrasound.
Quick recap: transverse = shakes across the travel direction (crests & troughs, can cross a vacuum); longitudinal = shakes along it (compressions & rarefactions, needs a medium).
Which Way Does a Particle Move Next?
A favourite exam question hands you a snapshot of a transverse wave, tells you which way it’s travelling, and asks which way a marked particle moves next. The trick: the wave shape just to the side the wave is coming from is about to arrive at that particle — so the particle’s next move copies the height of the wave just behind it.
🧭 Finding a particle’s next move
Note the wave type — for a transverse wave the particle can only move up or down
Note the travel direction of the wave from the arrow given
Look just “upstream” — at the part of the wave that will reach the particle next
The particle is about to take on that shape: if a crest is coming, it moves up; a trough, it moves down
WE 1
A snapshot shows a transverse wave travelling from right to left. A point P sits on the equilibrium line, with a crest just to its right. State the direction P moves immediately afterwards.
Reason it through
Transverse wave, so P can only move up or down
Use the travel direction
The wave moves right to left, so the shape on P’s right is arriving next
A crest is just to the right, so a crest is about to reach PP moves upwardsThe particle never moves left — only the wave pattern travels; P just rides up and down.
WE 2
A loudspeaker produces a sound wave that travels across a room.
(a) State the type of wave. (b) Describe how an air particle moves. (c) Explain why the sound cannot be heard in the vacuum of space.
Part (a)
Sound is a longitudinal wavePart (b)
Each air particle oscillates back and forth parallel to the direction the wave travels, about a fixed point
Part (c)
A longitudinal wave needs particles to pass on the compressions; a vacuum has none, so the wave cannot propagate
No medium, no sound — which is why space is completely silent.
💡 Top tips
Perpendicular = transverse, parallel = longitudinal — lock that pairing in
Only EM waves (and other transverse waves) can cross a vacuum; sound cannot
Particles never travel with the wave — they oscillate about a fixed point
For “which way next” questions, look at the wave shape on the side the wave is coming from
