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There are essentially two ways of transporting energy from the place where it is produced to that where it is intended to be utilized.
  1. The first involves the actual transport of matter. For example, a bullet fired from a gun carries its kinetic energy with it which can be used at another location.
  2. The second method by which energy can be transported is much more useful and important; it involves what we call a wave process. The wave carries energy but there is no transfer of matter.
When a drummer beats a drum its sound is heard at distant points. The sound carries energy as it can move the diaphragm of the ear. When a stone is dropped in the still water in a pond, water waves move steadily along the water until they reach the shore. If there is a small floating object, like a piece of cork, it will move up and down near its own location, which indicates that the molecules of water do not move along with the wave. When a bulb is turned on, the room is flooded with light. Light waves also carry energy. It is possible to transmit an electric signal (or a message) from one place to another. Although these various processes of transport of energy are different, yet they have a common feature which we will call wave motion.

The key word in wave motion is disturbance or perturbation. The reason why a variety of wave motions are possible is that there are various ways of disturbing the physical state of a body. In the case of water waves, the disturbance is the change in the position of water molecules relative to the equilibrium state (flat surface) brought about by dropping a stone. For sound waves, the changes of pressure can be regarded as a disturbance. For this disturbance to travel from one point of a medium to another, the particles of the medium must be coupled to one another by some force so that the disturbance created at one point can be handed down to its neighbours.

It is essential, at this stage, to clarify the meaning of the word ‘disturbance’. The scope of its meaning need not be limited, in a narrow sense, to mean the actual physical displacement of particles of a medium. In fact, a material medium is not even necessary if some physical property of space can exist in vacuum. We know that electric and magnetic fields can exist in vacuum. Disturbance, in this case, could be a perturbation (or change) in these fields. This disturbance travels in vacuum, needing no material medium for its propagation. Light waves are an example of such a wave.

In this chapter we will study only those waves which require a material medium for their propagation. We will now make an extremely important assumption, necessary for the existence of wave motion. We will ignore the molecular structure of matter and assume the medium to be continuous. The medium contains a large number of particles (e.g. one litre of air has 2.7 x 1022 molecules). If these are distributed within a limited space, the average distance between them becomes exceedingly small. As an approximation one can imagine the number of particles to be approaching infinity and intermolecular separation tending to zero. Such a system is said to be continuous. Implicit in this description is the existence of waves.

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