We have already seen that waves are produced in a medium due to such a disturbance which causes the particles of the medium to exhibit simple harmonic motion about their mean position one by one after a particular interval of time. In this way this disturbance is transmitted from one part of the medium to the other in the form of waves. We have already discussed two examples of waves. One was the waves produced on the surface of water and the other one was that of the waves produced in a stretched string. In both examples, waves need a medium for their production and propagation. In these examples, water and the string serve as a medium. Such waves are called “mechanical waves”.
There are such waves which do not require a medium for their propagation. Such waves are called electromagnetic waves Radio waves, television waves, X-rays, heat and light waves are the examples of such waves.
We will further discuss mechanical waves in this chapter. Mechanical waves are of two types:
(i) Transverse waves
(ii) Compressional or longitudinal waves.
The waves produced on the surface of water have been briefly examined in the experiment given at beginning of this chapter. You have seen that if one end of the pencil is dipped in the water and then moved up and down, waves are produced on the surface of water and due to the passage of these waves; the particles of water vibrate up and down. This means that the direction of vibratory motion of the water particles is perpendicular to the motion of the waves. Such waves are known as transverse waves. The waves produced by moving one end of a stretched string up and down are also transverse waves, because different parts of the string vibrate perpendicular to the string. Therefore, transverse waves can be defined as:
Transverse waves are such waves in which the particles of the medium vibrate perpendicular to the direction of propagation of the wave.
A sketch of the transverse waves is given in Fig. 12.5, which shows that when transverse waves pass through a medium, some of its particles move upwards while at the same instant some of them move downwards. Those parts of the transverse waves where the particles of the medium are above the normal position are called ‘crest’ and the parts where the particles of the medium are below their normal position are called ‘trough’.
The distance between two consecutive crests or troughs is called wavelength represented by Greek letter . A crest and a trough combine to make one complete wave. The number of waves passing through a point in one second is called the frequency of the waves. We will study in the next chapter about the sound waves in air which are longitudinal waves. The unit of frequency is ‘hertz’. The relation between frequency f and time period T is given as:
T=1/T …………. (12.6)
Compressional or Longitudinal Waves
The type of waves in which the direction of the vibratory motion of the particles of medium is parallel to the direction of propagation of the wave is called compressional or longitudinal waves. We will study in the next chapter about the sound waves in air which are longitudinal waves.
Category: 9th 10th