Longitudinal wave
A longitudinal wave is a type of wave where the particle displacement (the way they vibrate) is parallel to the direction of energy propagation.
More simply, the particles in the medium move back and forth in the same direction that the wave is travelling.
Compression and rarefaction
In a longitudinal wave, there are regions where the particles are:
- compressed together - known as compressions
- spread apart - known as rarefactions
When a longitudinal wave travels through a medium, it creates alternating compressions and rarefactions.
For example, sound waves in air are longitudinal waves. As the sound wave travels, it causes air particles to compress and then spread apart, creating variations in pressure that we hear as sound!
Examples of longitudinal waves
- Sound waves in air
- P-waves (primary waves) in earthquakes
- Ultrasound waves used in medical imaging