Wave motion: transverse and longitudinal waves
Key idea: Waves transfer energy while particles vibrate locally. Transverse vibrations are perpendicular to wave travel; longitudinal vibrations are parallel to it.
Offline pages
Saved on this device only. Connect at least once every 30 days to confirm Premium. Sign-out or a change of account removes saved copies.
Loading guide controls…
Guided lessons · coming soon
Guided lesson
Guided steps
- Track the ribbon or coil separately from the travelling wave.
- Predict what happens when the source stops.
- Follow energy from one part of the medium to the next.
- Explain how ripples make the float move.
- Find sound travel time and describe local particle motion.
- Compare vibration direction with wave travel.
- Compare spectator movement with the travelling stadium wave.
- Classify each wave using the two directions.
- Draw both arrows for the vertical spring, then classify its wave.
On this page
You stopped at:
ContinueRipples cross a pond while a floating leaf only bobs up and down; a sound reaches you across a room although the air particles only vibrate locally. A wave carries energy from place to place without carrying the material along with it: transfer by waves is one of the energy transfers in Energy stores and transfers.
Wave motion on ropes and springs
Tie one end of a long rope to a post and hold the other end. Move your hand up and down once: a hump, called a pulse, runs along the rope to the post. Keep moving your hand up and down steadily and a continuous wave travels along the rope.
A repeating wave starts with a vibrating source, here your hand. A single pulse needs only one disturbance. The rope is the medium: the material the wave travels through. Each piece of rope is pulled by the piece next to it, so it copies the movement of its neighbour a moment later. That is how the disturbance passes along.
Now tie a ribbon to the middle of the rope. As the wave passes, the ribbon moves up and down and returns to where it started. It does not travel to the post.
A long, loose spring (a slinky) shows the same idea in a different way. Stretch it along a smooth floor and push and pull one end along the length of the spring:
- Where the coils are pushed together, there is a compression.
- Where the coils are pulled apart, there is a rarefaction.
The compressions and rarefactions travel along the spring. A coil marked with tape moves backwards and forwards about its rest position; it does not travel along the spring either. If you instead shake the end of the spring from side to side, a wave like the rope’s travels along it.
Check your understanding 1
You stop moving your hand after making three waves on the rope. What happens to the three waves already on the rope?
Show answer
They keep travelling along the rope until they reach the post. Once a piece of rope is vibrating, it passes the movement on to its neighbour; the hand is only needed to start new waves. No new waves are made after your hand stops.
Waves transfer energy, not matter
A travelling wave transfers energy from the source to other places. It does not transfer matter: the particles of the medium vibrate about their mean positions.
The evidence is all around you:
- A leaf floating on a pond bobs up and down as ripples pass under it. The ripples reach the edge of the pond; the leaf stays near the same spot.
- The ribbon on the rope and the taped coil on the spring move to and fro about fixed positions.
- The waves still deliver energy: ripples can lift a floating leaf, a rope wave shakes the post at the far end, and sea waves wear away a cliff.
Moving your hand does work on the end of the rope. Each piece of rope then does work on the next piece, so energy passes along the rope from piece to piece, even though no piece travels with the wave. This is the transfer of energy by waves that you met among the energy transfers.
Worked example 1
What reaches a fishing float?
Problem
A fishing float rests on a lake with no significant current. Small ripples from a distant boat make the float bob up and down about the same place. Explain what the waves transferred to the float and what they did not.
Show full solution
What reaches a fishing float?
Method
Compare wave travel with local particle motion.Reason
A wave carries a disturbance and energy while particles oscillate locally.Working
Answer
- What travelled from the boat to the float? The disturbance, the wave.
- What did the water do? Each part of the water surface moved up and down about its own position and passed the movement on. The water near the boat did not travel to the float.
- So the waves transferred energy from the boat to the float: the float gained kinetic and gravitational potential energy as it bobbed. They did not transfer matter: the float still bobs about the same place after the waves have passed.
Check your understanding 2
A speaker is 3.4 m from a listener; sound travels at 340 m/s. Calculate its travel time. What happens to the air particles as the sound reaches the listener?
Show answer
The time is 3.4 ÷ 340 = 0.010 s. This is the travel time of the disturbance, not of the air beside the speaker. The air particles vibrate about their own positions and pass the vibration to neighbouring particles. Energy reaches the listener by the wave without that air travelling from the speaker to the ear.
Transverse and longitudinal waves
To classify a wave, compare two directions: the direction in which the particles of the medium vibrate, and the direction in which the wave travels.
- In a mechanical transverse wave, the particles vibrate at right angles (perpendicular) to the direction the wave travels. A transverse wave has crests and troughs.
- In a longitudinal wave, the particles vibrate along (parallel to) the direction the wave travels. A longitudinal wave has compressions and rarefactions.
| Transverse wave | Longitudinal wave | |
|---|---|---|
| Direction of vibration | perpendicular to the direction of travel | parallel to the direction of travel |
| What travels along | crests and troughs | compressions and rarefactions |
| Examples | waves on a rope or string; a spring shaken from side to side; water ripples; light, radio waves and all other electromagnetic waves | a spring pushed and pulled along its length; sound waves in air, water or solids |
For electromagnetic waves such as light, the changing electric and magnetic fields are transverse; no medium particles are needed.
The same spring can carry either kind of wave. What decides the kind is how the source vibrates compared with the direction the wave travels.
“Up and down” is not what makes a wave transverse. A rope lying on a table and shaken from side to side carries a wave whose vibrations are horizontal. They are still at right angles to the direction of travel, so the wave is still transverse.
Worked example 2
Classifying a stadium wave
Problem
Spectators in a stadium stand up and sit down one after another, so a “wave” runs sideways around the stands. Is it transverse or longitudinal?
Show full solution
Classifying a stadium wave
Method
Compare the vibration direction with the travel direction.Reason
Transverse and longitudinal describe the relationship between those two directions.Working
Answer
- Each spectator moves up and down, then returns to the same seat: that is the vibration.
- The wave moves horizontally around the stadium: that is the direction of travel.
- Up-and-down is at right angles to horizontal travel, so the stadium wave is transverse.
Check your understanding 3
Classify these waves: a guitar string’s points move sideways to the wave’s travel; rock is pushed and pulled along an earthquake wave’s travel; infrared travels from a heater to your hand.
Show answer
- The string wave is transverse: sideways vibration is perpendicular to travel along the string.
- The described earthquake wave is longitudinal: the rock moves along the wave’s direction of travel.
- Infrared is a transverse electromagnetic wave: its fields oscillate perpendicular to its travel.
Check your understanding 4
A long spring hangs vertically from a support. Its bottom end is pulled down and pushed up along the spring, and a wave travels up the spring. Draw a travel arrow and a two-headed vibration arrow on paper. Classify the wave using these directions, then compare your sketch below.
Show answer
Compare your arrows: the travel arrow points up the spring, while the two-headed vibration arrow points up and down along the spring. They lie along the same line. The wave is longitudinal. The coils vibrate up and down, along the same vertical line as the wave travels up the spring, so the vibration is parallel to the direction of travel. Up-and-down vibration does not by itself make a wave transverse.
Common mistakes
- Saying the water travels with the ripple. The water vibrates about its own position; the disturbance travels.
- Saying nothing moves in a wave. Each particle vibrates about its position and passes the movement on.
- Saying transverse means up and down. Transverse means the vibration is at right angles to the direction of travel, in any direction.
- Giving a longitudinal wave crests and troughs. It has compressions and rarefactions.
- Saying the waves stop when the source stops. Waves already made keep travelling; the source is needed only to make new ones.
Spotted an error? Report a correction
Syllabus and review details
- SEC G2 Science Physics component 2027 · 2027
Physics section, PDF pages 8–23
Last reviewed: