What is a wave?
Key idea: A wave carries energy without net movement of matter. In transverse waves the vibration is across the direction of travel; in longitudinal waves it is along it.
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Guided steps
- Press Play in Rope, then Sound. Follow the red particle and compare its motion with the travelling crest or compression.
- Explain how existing ripples reach the leaf after production stops.
- Draw two snapshots with separate travel and vibration arrows.
- Classify waves by comparing vibration with travel.
- Compare north–south vibration with eastward wave travel.
- Explain what the marked coil does as a compression passes.
- Explain why electromagnetic waves can cross a vacuum.
- Read wavefronts as matching stages of vibration.
- Draw both wavefront patterns and classify the changed orientations.
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Drop a stone into a pond and ripples spread out to the edge, while a leaf mainly bobs near one place when there is no current or wind. Something travels across the pond, but it is not the water. A wave is a travelling disturbance that carries energy from one place to another without carrying the material along with it. Ropes, springs and water ripples show how this works, and how waves divide into two kinds: transverse and longitudinal.
Waves carry energy, not matter
Move the end of a stretched rope up and down once and a single hump, a pulse, runs along it. Keep moving your hand up and down and you send a continuous wave. A repeating wave needs a vibrating source to keep producing it; a single pulse needs only one disturbance.
Tie a ribbon to one point of the rope. As the wave passes, the ribbon moves up and down and comes back to where it started; it never travels along the rope. Each piece of rope is pulled by the piece next to it, so the disturbance, and the energy your hand put in, is handed on along the rope while each piece only vibrates about its own rest position.
A stretched spring (a slinky) shows the same thing in a different way. Push and pull its end along its length and regions of squashed-together coils, called compressions, and spread-out coils, called rarefactions, travel along it. A coil marked with paint moves to and fro about one place; it does not travel with the compressions.
Follow the red particle in Rope and Sound: compare its local vibration with the crest or compression travelling right; sound is shown slowed down, with particle motion exaggerated.
A transverse wave on a rope: frequency 1.00 hertz, speed 2.00 metres per second. The ruler spans 1.00 metres.
- Particle equilibrium x
- 1.20 m
- Particle displacement y
- 0.088 m
- Tracked crest x
- 1.50 m
- Frequency, f
- 1.00 Hz
- Period, T
- 1.00 s
- Wave speed, v
- 2.00 m/s
- Ruler reading
- 1.00 m
- Distance, d
- 170.0 m
- Echo time, t
- — s
Your readings
| # | d / m | t / s | 2d / m | Remove |
|---|---|---|---|---|
| No readings yet. Set up a measurement, then record it. | ||||
In the simple small-ripple model, surface motion is local while the ripples spread. Real water particles can also move horizontally, and currents or wave-driven drift can carry a float; the no-net-transport model isolates wave propagation from those effects. Sound works like the spring: the air near a loudspeaker vibrates and passes the disturbance on, but that air does not flow all the way to your ear.
Check your understanding 1 The bobbing leaf
A boat stops making ripples while several ripples are still between it and a floating leaf. Must the leaf stop bobbing immediately? Explain what can still reach it after the source stops.
Show answer
No. Ripples already made can keep travelling and reach the leaf after the boat stops producing new ones. Their disturbance and energy make the leaf bob as they pass. The boat’s nearby water does not have to travel to the leaf, and no new ripples follow once the existing ones have passed.
Check your understanding 2 Construct the two motion accounts
Use the actual rope and spring figure. On paper draw an initial snapshot and a later snapshot for each wave travelling right. Keep a marked piece of rope or coil identifiable in both drawings; show a propagation arrow separately from its local vibration arrow. Label a crest and trough for the rope, and compression and rarefaction for the spring. A learner moves every coil to the next compression and says the coils carry the wave all the way along. Identify the first error and use your drawings to correct it.
Show answer
Self-review the rope drawings: crests and troughs move right, while a marked rope piece vibrates perpendicular to that travel about its own rest position. It need not be at the same displacement in the later snapshot. For the spring, crowding and spreading travel right, while the marked coil moves to and fro parallel to the spring about a local rest position. Label compression and rarefaction from spacing, not from the size of a coil.
The first error is treating a travelling region of compression as the same group of coils carried down the spring. Neighbouring coils pass the disturbance and energy on; a compression can reach distant coils without the original marked coil travelling with it. Draw a single propagation arrow and separate local double-ended vibration arrows, keeping each model’s material identities and sizes. These are schematic snapshots, not measured individual paths.
Transverse and longitudinal waves
Waves are sorted by comparing two directions: the direction in which the particles vibrate, and the direction in which the wave travels.
- In a transverse wave, the particles vibrate perpendicular to the direction the wave travels. The rope wave is transverse.
- In a longitudinal wave, the particles vibrate parallel to the direction the wave travels. The spring wave is longitudinal, and so is sound.
| Feature | Transverse | Longitudinal |
|---|---|---|
| Vibration compared with travel | perpendicular | parallel |
| What travels along it | crests and troughs | compressions and rarefactions |
| Examples | waves on a rope, water ripples, light and all electromagnetic waves | sound, a spring pushed and pulled along its length |
“Transverse” does not mean “up and down”. A rope shaken from side to side on a table carries a transverse wave too, because the vibration is still at right angles to the direction of travel.
Worked example 1
A wave on a rope on a table
Problem
A wave travels east along a rope lying on a table. A taped point on the rope moves north and south. Is the wave transverse or longitudinal?
Show full solution
Compare the two directions
Method
Compare the direction of vibration with the direction the wave travels.Reason
The type of wave depends only on the angle between these two directions, not on which way is up.Working
Answer
The point vibrates north–south; the wave travels east.
Classify the wave
Working
Answer
North–south is perpendicular to east, so the wave is transverse, even though nothing moves vertically.
Quick check
A slinky pushed and pulled along its length, east and west, would carry a longitudinal wave instead.
Check your understanding 3 Following one coil
A compression travels to the right along a spring. Does a marked coil travel steadily to the right with it? What kind of wave is this?
Show answer
No. The coil moves backwards and forwards about its rest position, along the length of the spring. The compression is what travels. Because the coils vibrate parallel to the direction of travel, it is a longitudinal wave.
Waves that need a medium, and waves that do not
Waves on a rope, water ripples and sound are mechanical waves: they travel by making the particles of a material (the medium) vibrate, so they cannot cross a vacuum. Light, radio waves and the rest of the electromagnetic waves need no medium; light reaches us from the Sun through the near-vacuum between planets. For electromagnetic waves, transverse describes the field oscillations, not vibration of material particles in empty space.
Wavefronts
Ripples spreading on water are easiest to picture from above as a set of lines. A wavefront is a line joining points of a wave that are at the same stage of their vibration at the same moment, such as all the points along one crest. Neighbouring crest wavefronts are one wavelength apart.
The shape of the wavefronts depends on the source:
- a vibrating straight bar dipped in water makes straight wavefronts;
- a small vibrating dipper makes circular wavefronts.
In this uniform-depth model, propagation is perpendicular to the wavefronts. These arrows show travel of the disturbance, not transport of the water.
The ripple-tank lesson shows how wavefronts are made visible and used to measure a wave.
Check your understanding 4 Construct wavefronts and classify changed orientations
Use the actual wavefront figure as a reference, then construct your own plan views for a straight bar and a point dipper in uniform-depth water: draw neighbouring crest wavefronts and outward propagation arrows at two different positions. Label what one line joins and what neighbouring crest spacing represents.
In a separate rope wave, propagation is north and a marked point vibrates east–west. In a vertical spring, propagation is upwards and coils vibrate up–down. Draw and compare the two directions in each case to classify the waves. Explain why “up–down always means transverse” and “every wave needs vibrating material” are both too broad.
Show answer
For the bar, draw approximately parallel straight crest lines with arrows perpendicular to them away from the source. For the point dipper, draw concentric crest circles and arrows radially outwards at different positions; one fixed arrow direction does not describe the whole circle. Each wavefront joins points at the same stage of vibration at one instant. Adjacent crest fronts are one wavelength apart; the figure has no scale for measuring a wavelength. The arrows represent propagation, not water flowing outwards.
The north-travelling rope with east–west vibration is transverse: the directions are perpendicular. The upward-travelling vertical spring with up–down vibration is longitudinal: they are parallel. Compare both arrows, rather than using vertical as a definition. Mechanical waves need a material medium; electromagnetic waves can propagate through a vacuum, with transverse field oscillations rather than material particles crossing it.
Common mistakes
- Saying the rope or water travels along with the wave. Only the disturbance and its energy travel.
- Saying “transverse means vertical”. Compare the vibration with the direction of travel.
- Saying all waves need air. Mechanical waves need a medium of some kind; electromagnetic waves need none.
- Defining a wavefront as just “a line on the water”. It joins points at the same stage of vibration at the same moment.
- Forgetting to give both directions when classifying a wave.
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