The waves so far discussed can be:
(a) Transverse waves
(b) Longitudinal waves.
(a) Transverse waves:
The waves in which each particle of the medium executes vibrations about its mean position and is perpendicular to the direction of the waves are called transverse waves. For example, the waves generated in a pond when a stone is thrown, all electromagnetic waves, stretched string of sitar, violin, etc.
(b) Longitudinal waves:
The waves in which each particle of the medium executes vibrations about its mean position, in the direction of the propagation of the wave, are called longitudinal waves. For example, sound waves in air, sound waves inside water.
Longitudinal waves can also be defined as the waves in which the medium particles have periodic change in displacement and pressure in the direction in which they travel.
In a longitudinal wave, if the particles of the medium vibrate horizontally (the direction of movement of the tuning fork), then the disturbance also travels horizontally. Again this wave travels in the form of compression and rarefaction.
A compression is the region or a space of the medium, in which the particles come close to a distance less than the normal distance between them.
A rarefaction is the region or a space of the medium, in which the particles get apart to a distance greater than the normal distance between them.