Wave motion: the energy travels, the material stays behind

1. Every wave starts with a source that does work on something: your hand flicks a rope, a loudspeaker cone pushes the air, a dipper taps the water surface.
2. That first disturbance drags its neighbouring bit of material with it, which drags the next bit, and so the disturbance is handed on from one part of the material to the next.
3. Each part of the material moves away from its rest position and then comes back to it, so once the wave has gone past, that part is exactly where it began.
4. What moves steadily forward is the pattern of disturbance, and with it the energy the source supplied.
5. You can tell the energy has arrived because it does work at the far end: a water wave rolls pebbles up a beach, and a sound wave sets your eardrum vibrating.
6. The bigger the disturbance the source makes, the more energy the wave carries, which is why a storm wave does far more damage than a ripple travelling at the same speed.

Ropes and springs make wave motion slow enough to watch

1. Lay a long rope across the floor, hold one end and flick it sharply up and back down again.
2. A single hump, called a pulse, races away from your hand towards the far end.
3. Tie a bright ribbon to the middle of the rope and the test becomes obvious: the ribbon flicks up and down as the pulse goes by, then hangs still in the same place it started.
4. Keep moving your hand up and down instead of flicking once, and you get a continuous wave instead of a single pulse, but the ribbon still just vibrates on the spot.
5. A stretched spring shows the same behaviour, and it is easier to see because you can watch individual coils.
   1. Shake one end from side to side and a sideways wiggle runs along the coils to the far end.
   2. Push and pull one end along the spring's own length instead, and a moving pattern of squashed and stretched coils travels along instead.
6. Mark one coil with a dab of paint and, in both cases, it ends up back where it started once the wave has passed.
7. This is why wave motion is described in terms of vibrations: a vibration is a repeated to-and-fro movement about a rest position, and that is all any one piece of the material ever does.

Water waves carry energy across the surface, not the water itself

1. In a ripple tank, a motor vibrates a bar or a small dipper that just touches the water, and ripples spread steadily outwards from it.
2. Drop a small piece of cork or a leaf onto the surface and it bobs up and down on the spot as each ripple passes underneath it.
3. The cork does not get swept to the far side of the tank, which is the direct evidence that the water is not being transported along with the wave.
4. Each patch of water is lifted and lowered by its neighbour, passing the disturbance on and then settling back to the same place.
5. On a much larger scale, wind blowing on the Atlantic can send waves thousands of kilometres to break on a Cornish beach, but the water that breaks there is not water blown over from America.
6. What crosses the ocean is energy, and the beach feels it as the waves break and shift the sand and shingle.