Explain the types of waves on the basis of direction of oscillations of particles of medium.

Explanation

Depending upon the direction of oscillations of particles of medium, mechanical waves are classified into following two categories
: (1) Transverse waves (2) Longitudinal waves.
(1) Transverse Waves : "During the propagation of given mechanical waves, if particles of medium oscillate perpendicular to the direction of propagation of wave then they are called transverse waves."

Consider a horizontal string tied at one end with the fixed rigid support which is stretched and kept under proper tension as shown in fig (a). Now give a little single jerk at the free end of this string on upper side of X-axis such that first particle of string goes up slightly from origin and then comes back to the origin. (i.e. first particle undergoes half oscillation.) By doing so, we notice that a little crest is formed (called pulse) which starts moving away from first particle, causing up and down motion of neighbouring particles. Here we notice that as the single crest produced initially at the free end moves away, its size (i.e. height at the centre of crest) goes on decreasing. This is called damping of a pulse.
If length of string is very large as compared to initial size of pulse then it will get completely damped out before it reaches opposite end and so its reflection from that end may be ignored. But if the string is short and pulses are produced continuously at the free end then they get reflected from the opposite end. Here if initial pulse is in the from of a trough then it travels in the form of a trough.

Now, consider fig. (b) in which one full oscillation is given to the first particle at the free end. At the end of this oscillation, we find that a sinusoidal wave shape is produced which contains one trough and one crest. Now as this wave shape moves ahead, it gets damped. Here also if string is very long as compared to height of crest or depth of trough, then its reflection can be ignored from the opposite end.
But if the string is short and oscillations are produced continuously at the free end then wave shapes are also produced continuously one after another which propagate away from free end. Such oscillations give rise to a "Wavetrain."
In both the cases of fig. (a) and fig. (b) given above, particles of string oscillate perpendicular to the direction of propagation of wave. Hence according to definition, these string waves are transverse waves.
(2) Longitudinal Waves: "During the propagation of given mechanical waves, if particles of medium oscillate parallel (or antiparallel) to the direction of propagation of wave then they are called longitudinal waves."

As shown in the figure, consider a long pipe filled with air having piston at one end. Now when we push the piston forward and then pull it backward suddenly, condensations and rarefactions are generated in the air column. If we push & pull the piston continuously and sinusoidally, the disturbance produced in the air column at the top end moves away from it, parallel to the length of the pipe, which produces sound. Hence the waves propagating in air column are called sound waves.
Here the air particles inside the pipe oscillate parallel (or antiparallel) to the direction of propagation of sound waves and hence according to definition, sound waves are longitudinal waves.