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Transverse and Longitudinal Waves

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Transverse and Longitudinal Waves - Lesson Summary

Mechanical waves can be either transverse or longitudinal.
 
In a transverse wave the particles of the medium vibrate in a direction normal to the direction of the propagation of the wave.
 
In a longitudinal wave the particles of the medium vibrate in a direction parallel to the direction of the propagation of the wave.
 
In both the waves it is the disturbance that propagates and not the particles of the medium. The particles of the medium only vibrate about their mean positions.
 
To understand how a transverse wave is produced and propagated, first fix one of the ends of a string of length of 2 meters to a pole. Hold the free end of the string tight so that the string is in tension. Now if the hand is moved up and down about a mean position with a simple harmonic motion, a continuous wave which is sinusoidal in shape travels along the string with a velocity v. Now mark a point on the string and observe as the waves propagate through the string to study the wave formed in this process. We see that the point marked moves up and down the mean position.
 


 
Apparatus required understanding the   production and propagation of a longitudinal wave is a glass cylinder with an open end. The open end of the cylinder is provided with an air tight piston. Air with lots of dust particles is trapped inside this cylinder.
 
Now let us place this cylinder in a dark room with a focus light behind the cylinder as shown in the figure.
 


The piston is pushed inside the cylinder and pulled back with a jerk. We now observe the dust particles near the piston are compressed and form a band. This is due to increase in pressure created by the piston on the particles. This band, called compression moves towards the closed end of the cylinder as a pulse. When the piston is pulled back, we observe that the pressure at the compressed region decreases and causes the elements of air to moves backwards from the compression. The low pressure region formed is called rarefaction.
 
Now let us move the piston forward and backward about a mean position with a simple harmonic motion. We see a series of compression and rarefaction bands formed by the dust particles inside the cylinder moving towards the closed end.
 
In a transverse wave, as the wave propagates each element of the medium undergoes a shearing strain. Therefore transverse wave can propagate through solids and liquids.
Waves formed on the surface of a sea are transverse waves called gravity waves whose wavelength is of the order of metres. The restoring force in these waves is the gravitational pull on the water.
 
 
In a longitudinal wave, as the wave propagates each element of the medium undergoes a compressive strain. Therefore a longitudinal wave can propagate through solids, liquids and gases

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