Why are ripples circular, even if the stone that causes them isn't?

14 September 2008

Question

I had an L-shaped stone, threw it in the water and but all the ripples were circular. Why is that?

Answer

Chris - It's a common thing, isn't it? When you throw things in you think why do I get this ripple going out? The answer is that when you first throw a stick into water you will get a stick-shaped initial ripple but as the ripples spread out you'll get the L-shaped bit going out a bit but then what about the spaces? What about the bits in between? They'll get filled in with a curve. Eventually as it gets farther and farther out you'll end up with something that is predominantly curve with very little of the original shape left behind. It looks to all intents and purposes like a giant circle. The contribution of the original shape is absolutely tiny. That is why it does that interesting morphing thing into a circle from something which was originally a different shape.

Dave - So all the ripples are going outwards at the same speed and they start off in different positions but after a couple of seconds those different positions they start on doesn't make much of a difference. Chris - In the grand scheme of things, no.

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