Science Questions

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

Sun, 14th Sep 2008

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Mark Wolford asked:

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


Chris -  Itís a common thing, isnít 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 youíll get the L-shaped bit going out a bit but then what about the spaces?  What about the bits in between?  Theyíll get filled in with a curve.  Eventually as it gets farther and farther out youíll 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 doesnít make much of a difference.

Chris - In the grand scheme of things, no.


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Mark Walford asked the Naked Scientists: Hi, Whilst on a beach recently, I threw in an L shaped lump of stone...but the resultant ripples were still circular - how does this happen? Many thanks, Mark What do you think? Mark Walford , Thu, 11th Sep 2008

The ripples will start off as the shape of the object, assuming that the object impacts the water in a way such that impact with the surface represents the shape. By this I mean if you throw in a long straight stick, we are speaking of the stick hitting the water with both ends roughly at the same time and not, at the other extreme, with one end hitting (end on) and the other end following the same linear trajectory. The geometry of how the waves combine with irregular shapes is a little complex but, in general, the further the waves travel away the closer they will appear circular. If we take the case of the long straight stick, you would expect the waves to start out as a thin rectangle. But as the waves move away from every point on the stick at the same speed, this turns out as two flat waves from either side but semicircles at either end. As time goes on this shape will be roughly maintained. However if you were to view this from a distance where the length of the stick is small compared with the size of the spreading wave, it starts to look more circular. You can draw this out on a piece of paper. The linear waves from the sides stay the same length as the stick but the two semicircles from the ends get bigger so that it looks increasingly like a circle.

With an L-shape the waves from the two sides that face each other at right-angles will interfere a bit but the same principle applies. From a big enough distance all shapes look like a point and so the waves will tend to look circular. graham.d, Thu, 11th Sep 2008

Just to add to what Graham has said:, Thu, 11th Sep 2008

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