In general all waves mechanical and electromagnetic (non-mechanical) waves are classified into two types. These are

• longitudinal waves

• transverse waves.

Longitudinal waves

Propagation of sound waves in air

A wave motion in which the particles of the medium oscillate about their mean positions in the direction of propagation of the wave, is called longitudinal wave.

Sound waves are classified as longitudinal waves. Let us now see how sound waves propagate. Take a tuning fork, vibrate it and concentrate on the motion of one of its prongs, say prong A. The normal position of the tuning fork and the initial condition of air particles is shown in the fig (a). As the prong A moves towards right, it compresses air particles near it, forming a compression as shown in fig (b). Due to vibrating air layers, this compression moves forward as a disturbance. As the prong A moves back to its original position, the pressure on its right decreases, thereby forming a rarefaction. This rarefaction moves forward like compression as a disturbance. As the tuning fork goes on vibrating, waves consisting of alternate compressions and rarefactions spread in air as shown in fig (d). The direction of motion of the sound waves is same as that of air particles, hence they are classified as longitudinal waves. The longitudinal waves travel in the form of compressions and rarefactions.

Transverse waves

A wave motion, in which the particles of the medium oscillate about their mean positions at right angles to the direction of propagation of the wave, is called transverse wave.

These waves can propagate through solids and liquids but not through gases, because gases do not possess elastic properties. Examples of these waves are: vibrations in strings, ripples on water surface and electromagnetic waves.

In a transverse wave the particles of the medium oscillate in a direction perpendicular to the direction of propagation as shown in the figure.

Particles of the medium oscillate in a direction perpendicular to the direction of propagation

Thus, during their oscillations, the particles may move upwards or downwards from the plane passing through their mean positions. The uppermost point of the wave, i.e., the position of maximum positive displacement is crest and the lowest point, i.e. the position of maximum displacement is called trough. Thus in a transverse wave crests and troughs appear alternatively.

From : xinlei:)