Longitudinal and transverse waves

In longitudinal waves, the vibrations are parallel to the direction of wave travel.

Examples of longitudinal waves include:

  • sound waves
  • ultrasound waves
  • seismic P-waves

One way to remember the movement of particles in longitudinal waves is to use the ‘P’ sound: longitudinal waves such as seismic P-waves may be thought of as pressure or push waves as the particles move parallel to the wave.

Demonstrating longitudinal waves

Longitudinal waves show areas of compression and rarefaction:

  • compressions are regions of high pressure due to particles being close together
  • rarefactions are regions of low pressure due to particles being spread further apart
An outstretched slinky spring

In the diagram, the compressions move from left to right and energy is transferred from left to right. However, none of the particles are transported along a longitudinal wave. Instead, they move backwards and forwards between compressions as the wave is transmitted through the medium.

Transverse waves

In tranverse waves, the vibrations are at right angles to the direction of wave travel.

Examples of transverse waves include:

  • ripples on the surface of water
  • vibrations in a guitar string
  • a Mexican wave in a sports stadium
  • electromagnetic waves – eg light waves, microwaves, radio waves
  • seismic S-waves

One way to remember the movement of particles in transverse waves is to use the ‘S’ sound: transverse waves such as seismic S-waves may be thought of as shake or shear waves as the particles move from side-to-side – crossing the direction of wave travel.

Demonstrating transverse waves

Transverse waves are often demonstrated by moving a rope rapidly up and down.

A hand holds a length of rope taut

In the diagram the rope moves up and down, producing peaks and troughs. Energy is transferred from left to right. However, none of the particles are transported along a transverse wave. The particles move up and down as the wave is transmitted through the medium.