Ripples on water

Drop a stone into still water and ripples spread out from where it fell, in circles that share one centre.

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Figure 4.1 — circular ripples on a water surface, spreading outward from the point where a drop fell.

The stone gives the water a disturbance at one point, and that disturbance does not stay there. It travels outward as the ripples.

A rope does the same

Hold one end of a rope and shake it up and down. Ripples run along the rope too. Here the disturbance is made by your hand, and it travels along the rope — your hand acts as the source of the ripples.

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Figure 4.2 — a hand moving one end of a rope up and down, with ripples travelling along the rope away from it.

Wave

A disturbance that travels through a medium or through space is called a wave.