Definition
Transverse wave
A transverse wave is one in which the oscillations are perpendicular to the direction of energy transfer.
Definition
Longitudinal wave
A longitudinal wave is one in which the oscillations are parallel to the direction of energy transfer.
- All waves transfer energy from one place to another without transferring matter overall.
- Waves are classified as transverse or longitudinal by the direction of oscillation compared with the direction of energy transfer.
- Ripples on a water surface are transverse: the surface moves up and down while energy travels sideways across the water.
- In a longitudinal wave, each particle vibrates backwards and forwards about a fixed position, in the same line as the wave travels.
Crests and troughs
- A transverse wave shows a repeating pattern of high points called crests and low points called troughs.
- A crest is the highest point of the wave above the undisturbed (rest) position, and a trough is the lowest point below it.
- Examples of transverse waves include ripples on a water surface, waves on a rope or string and all electromagnetic waves, such as light.
- Mechanical waves, such as water ripples, need a medium to travel through, whereas electromagnetic waves are also transverse but can travel through a vacuum, so they need no medium.
Compressions and rarefactions
- A longitudinal wave shows alternating regions called compressions and rarefactions.
- A compression is a region where the particles are pushed closer together.
- A rarefaction is a region where the particles are further apart.
- Sound waves travelling through air are longitudinal.
- A vibrating loudspeaker cone pushes air particles together and then pulls back, sending a train of compressions and rarefactions through the air.
Key Idea
In a sound wave, the air particles oscillate parallel to the direction in which the sound energy travels.
The wave travels, not the material
- A wave transfers energy but does not carry the material it passes through along with it.
- A floating cork on water only bobs up and down as a ripple passes, showing the water does not travel across the surface with the wave.
- Sound crosses a room without any steady flow of air across it, because the air particles only vibrate about their positions while the compressions and rarefactions travel to the listener.
Example
Describe the difference between a transverse wave and a longitudinal wave.
- In a transverse wave, the oscillations are perpendicular to the direction of energy transfer.
- In a longitudinal wave, the oscillations are parallel to the direction of energy transfer.
- A longitudinal wave also has compressions and rarefactions, whereas a transverse wave has crests and troughs.
Common Mistake
- Do not say the particles travel with the wave; they oscillate about fixed positions while the energy moves through the material.
- Do not confuse the direction of oscillation with the direction of wave travel: they are perpendicular for a transverse wave and parallel for a longitudinal wave.
Exam technique
- When comparing the two wave types, always refer to both the direction of oscillation and the direction of energy transfer.
- Vague phrases such as "up and down" are not enough on their own, because the wave may be travelling in a different direction.
- For evidence that matter is not carried along, state what is observed and then say what it shows.
Self review
- How are the oscillations arranged in a transverse wave compared with the energy transfer?
- How are the oscillations arranged in a longitudinal wave?
- Which type of wave is a ripple on water, and which is a sound wave in air?
- What is a compression and what is a rarefaction?
- How does a floating cork show that water does not travel across the surface with a ripple?
