Waves  (Page 2/6)

4: The oscillatory motion of preceding particle is imparted to the adjacent particle following it.

Returning to real time ripples on the water surface, we observe that ripples are originating continuously from the contact point for some time. It is the case of multiple disturbances. As such, a succession of ripple passes through water surface. The particle at the crest moves down to form trough subsequently and vice-versa. This cycle continues till the disturbance at the origin is over. Conclusion :

5: We need to keep creating disturbance in order to propagate wave (energy or disturbance) continuously.

Wave and medium

Water wave and propagation of sound waves show the importance of wave transmitting medium. These waves move through a medium. The Maxwell description of electromagnetic waves, however, puts an end to the requirement of a medium for the propagation of waves in general. The light wave (electromagnetic wave), for example, travels most of its journey from Sun to Earth in vacuum! The propagation of electromagnetic wave does not require a medium for propagation. On the other hand, we are unable to talk on Moon as it does not have its atmosphere to carry sound wave. Thus, we conclude that the wave can move with or without medium depending on the specific wave in question.

Disturbance

Disturbance is alteration of physical property in time. This physical quantity can be displacement, pressure, density, electric or magnetic field etc. In the propagation of water waves, we denote relative position or displacement of water molecules from the mean position as disturbance. Similar is the case with string wave – the wave traveling through a taught string. Here, displacement of string element from the undisturbed position is used to measure disturbance. We account the disturbance by air pressure in the case of sound. There is change in stress values in the case of seismic waves through Earth. On the other hand, the electromagnetic wave involves simultaneous alteration of electric and magnetic fields at right angle to each other and also to the direction of wave propagation

The most general physical parameter that can be used to represent alteration in all cases is “energy”. Instead of a particular physical parameter in specific cases, we can consider them to be alterations of “energy”. If required, we can specify the type of energy as well like : mechanical or elastic energy, pressure energy or electromagnetic energy as appropriate for a specific wave.

Dimensions of waves

The wave in a taught string moves along a straight line and as such is one-dimensional. The water wave moves over the two dimensional surface of water body and hence water wave is two-dimensional. On the other hand, sound waves or light waves propagate all around the source and hence are three dimensional in nature.

Wave classification

The waves are classified under three high level headings :

1: Mechanical waves : The motion of the particles constituting the medium follows mechanical laws i.e. Newton’s laws of motion. A medium is required for propagation of mechanical waves. The motion of the waves depends on the mechanical properties of the medium.