As per physics definition a wave is a periodic disturbance which propagates energy from place to other. One of the important type of wave is mechanical wave and it need materiel medium for propagation.
Mechanical waves are further classified depending on the space up to where they propagate.If the wave propagate for infinite distance and never come back then it is called progressive wave.If the wave is confined between any two finite points then it is called stationary wave.
A progressive wave is further classified into two types basing on the vibration of particles and direction of motion of wave.
If the direction of vibration of particles of elastic medium is parallel to direction of wave propagation it is called longitudinal wave and if the vibration of particles is perpendicular to direction of wave motion then the wave is called transverse wave.
The following is the representation of longitudinal progressive wave and we can notice the compressions and rarefactions . A compression is a place where particles are crushed close and rarefaction is a place where particles are bit separated.
The compressions and rarefactions are systematic and they are separated by fixed distance.Two successive compressions and rarefactions are constantly separated by a fixed distance and it is called wave length.
Further we can draw a case of transverse wave as shown below.
In transverse waves, the particle motion is normal to the direction of propagation of the wave. Therefore, as the wave propagates, each element of the medium undergoes a shearing strain. Transverse waves can, therefore, be propagated only in those media which can sustain shearing stress, such as solids and strings, and not in fluids.Fluids as well as solids can sustain compressive strain; therefore, longitudinal waves can propagate in all elastic media. For example, in medium like a steel bar, both transverse and longitudinal waves can propagate while air can sustain only longitudinal waves.
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