Waves  (Page 3/6)

2: Electromagnetic waves : These are propagation of electric and magnetic fields. The electromagnetic waves do not require a medium for propagation. Its speed in vacuum is a universal constant. The motion of the electromagnetic waves in a medium depends on the electromagnetic properties of the medium.

3: Matter waves : The matter wave is associated with all matter. The wave nature of matter is not always appreciable. It becomes appreciable for sub-atomic particles like electrons, which display significant wave properties. This is the basis of electron microscope.

Nature of waves

The disturbance (vibration) and direction of wave may be either in the same direction (longitudinal wave) or mutually perpendicular (transverse wave). Primary seismic waves (also known as P-waves) and sound waves are examples of longitudinal waves. Electromagnetic waves are transverse waves in which electric and magnetic fields are alternating at right angles to the direction of propagation. Water waves, as discussed earlier, is combination of longitudinal and transverse waves.

Transverse waves

Transverse waves are characterized by the relatigve directions of vibration (disturbance) and wave motion. They are at right angle to each other. It is clear that vibration in perpendicular direction needs to be associated with a “restoring” mechanism in transverse direction.

Consider a sinusoidal harmonic wave traveling through a string and the motion of a particle as shown in the figure above (only one unit of wave shown for illustration purpose). Since the particle is displaced from its natural (mean) position, the tension in the string arising from the deformation tends to restore the position of the particle. On the other hand, velocity of the particle (kinetic energy) moves the particle farther away from the mean position. Ultimately, the particle reaches the maximum displacement when its velocity is zero. Thereupon, the particle is pulled down due to tension towards mean position. In the process, it acquires kinetic energy (greater speed) and overshoots the mean position in the downward direction. The cycle of restoration of position continues as vibration (oscillation) of particle takes place.

For mechanical wave through solid, the restoring mechanism is provided by the elastic property of the medium. A body of liquid does not display elastic property. It can not restore deformation caused by shear i.e. transverse force. As such, transverse wave can not pass through a liquid body. On the surface of liquid, however, transverse (perpendicular to surface) deformation is restored by surface tension. For this reason, water surface waves have a transverse component. It means that transverse waves can be sustained on the surface, but not within the body of the liquid.

As far as gas is concerned, it does not exhibit elastic property like solid or surface tension like liquid. Gas simply yields to slightest transverse force without being restored. The intermolecular forces between molecules are too feeble to restore transverse deformation. Thus, mechanical transverse waves can not propagate through gas at all. Electromagnetic waves are exception to this. It is because EM waves do not require medium. There is no need to restore the system mechanically.