Waves can be described as oscillations or vibrations about a rest position. For example:

  • sound waves cause air particles to vibrate back and forth;
  • ripples cause water particles to vibrate up and down.

The direction of these oscillations is the difference between longitudinal and transverse waves.

Longitudinal waves

A longitudinal wave is one in which the vibrations of the particles are parallel to the direction in which the energy of the wave travels.

Demonstrating longitudinal waves

Longitudinal waves show areas of compression and rarefaction:

  1. compressions are regions of high pressure due to particles being close together.
  2. rarefactions are regions of low pressure due to particles being spread further apart.
A hand is holding a stretched metal coil.

1. An outstretched slinky spring

In the diagram, the compression moved from left to right and energy is transferred from left to right.

The movement of the coils of the slinky and the energy are parallel.

However, none of the particles are transported along a longitudinal wave.

Instead, they move backwards and forwards as the wave is transmitted through the medium.

Examples of longitudinal waves include:

  • sound waves
  • ultrasound waves
  • P type earthquake waves

One way of remembering examples of longitudinal waves is to emphasise the O in lOngitudinal and sOund and ultrasOund.