Why are ripples circular, even if the stone that caused them is not?

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Mark Walford

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Mark Walford  asked the Naked Scientists:

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,

What do you think?


Offline graham.d

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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.



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