PHYSICAL CHARACTERISTICS OF WAVES

When we observe a wave it is clear that something, that we may call a disturbance, travels or propagates from one region of a medium to another. This disturbance travels at a definite velocity v that is usually determined by the mechanical properties of the medium. For a taut string these are the mass per unit length and the tension in the string. However, the medium does not travel with the wave.
For example, if we tap one end of a solid metal rod, a sound wave propagates along the rod but the rod itself does not travel with the wave. (For this reason, waves can travel at high velocities.) In fact, the particles of the rod move about their equilibrium positions to which they are bound. We saw such behaviour for the transverse oscillations of masses connected by springs. There the equilibrium position was the straight line along which the masses lie when at rest and the springs provide the restoring force. An oscillator can transfer all of its energy to another oscillator to which it is coupled under appropriate conditions. A simplified picture of a wave travelling through a medium is therefore a long line of oscillators coupled together in some way, just like the atoms in a one-dimensional crystal. Then if the end oscillator is displaced from its equilibrium position it exerts a force on its neighbour. In turn this force and the resultant displacement propagate down the line of oscillators. Energy must be put into the system to cause the initial disturbance and it is this energy that is transmitted by the wave. This energy is evident as the destructive power of a tsunami and in the warmth of the Sun’s rays. On a sunny day the solar energy deposited on the Earth’s surface is about 1 kJ m⁻² s⁻¹; a power of 1 kWm⁻². This is a substantial amount of power that is an increasingly important source of energy for the World’s needs.