It is a kind of wave motion in which the individual particles of a medium execute periodic motion about their mean position along the direction of propagation of wave. Let us consider a pulse moving down a long stretched spring as shown in figure. The left end of the spring is pushed briefly to the right and then pulled briefly to the left. This movement creates a sudden compression in a region of the coils. The compressed region travels along the spring (to the right in figure). Notice that the direction of the displacement of the coils is parallel to the direction of propagation of the compressed region.
The longitudinal wave along a stretched spring. The displacement of the coils is in the direction of the wave motion. Each compressed region is followed by a stretched region.
Sound waves are example of longitudinal waves. The disturbance in a sound wave is a series of high-pressure and low-pressure regions that travels through air.
A longitudinal wave moves by the phenomena of compression and rarefaction in the medium.
In compression, distance between any two consecutive particles of medium is less than their normal distance. Therefore, density is more than the normal density. In a rarefaction, distance between any two consecutive particles of medium is more than the normal distance, therefore, density is less than the normal density.
- At an instant of time a particle in compression phase and another in rarefaction phase are exactly out of phase from each other.
- All particles in compression phase are in phase (at a given instant of time).
- All particles in rarefaction phase are in phase (at a given instant of time).
Distance between two consecutive compressions or rarefactions is equal to the wavelength of the wave. Longitudinal waves can propagate through any state of matter.