More familiar types of waves are sound, or waves on a surface of water. In both cases, there is a perturbation with a periodic spatial pattern which propagates , or travels in space. In the caseof sound waves in air for example, the perturbed quantity is the pressure, which oscillates aboutthe mean atmospheric pressure. In the case of waves on a water surface, the perturbed quantityis simply the height of the surface, which oscillates about its stationary level. Figure 1.1 showsan example of a wave, captured at a certain instant in time. It is simpler to visualize a wave bydrawing the “wave fronts”, which are usually taken to be the crests of the wave. In the case of Figure 1.1 the wave fronts are circular, as shown below the wave plot.