2.1 What are waves?

Waves are disturbances which propagate (move) through a medium.¹ Waves can be viewed as a transfer energy rather than the movement of a particle. Particles form the medium through which waves propagate but they are not the wave. This will become clearer later.

Let's consider one case of waves: water waves. Waves in water consist of moving peaks and troughs. A peak is a place where the water rises higher than when the water is still. A single peak or trough we call a pulse. A wave consists of a train of pulses.

So waves have peaks and troughs. This could be our first property for waves. The following diagram shows the peaks and troughs on a wave.

In physics we try to be as quantitative as possible. If we look very carefully we notice that the height of the peaks above the level of the still water is the same as the depth of the troughs below the level of the still water. The size of the peaks and troughs is the same.