The Duck That Goes Nowhere

Throw a stone into a still pond. Rings spread outwards, wider and wider, until they reach the far bank.

Now suppose a duck is floating out there. The rings reach it. What happens to the duck?

Most people picture it being carried along, pushed towards the far side. It isn't. The duck goes up, and then down, and stays exactly where it was. The waves pass underneath and keep going. The duck does not travel with them.

That's worth sitting with for a moment, because it tells you what a wave actually is. The water didn't move across the pond. Something moved across the pond — you saw it happen — but it wasn't the water. What travelled was the up-and-down, handed from one bit of water to the next, exactly the way a push was handed from one particle to the next in the last lesson.

A wave carries the shaking. It does not carry the stuff.

By the end of this lesson you'll be able to:

• name the crest, the trough, the wavelength and the amplitude of a wave
• explain what one full wave means, and what frequency counts
• work out a wave's speed from its frequency and its wavelength
• explain why the duck bobs but doesn't move along