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I have had a question since childhood. Why do we always get circular waves (ripples) in water even when we throw irregularly shaped object in it?

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I suppose that irregularly shaped object is small compared with the ripples? – KennyTM Nov 25 '10 at 6:55
Perhaps you got my question wrong. I have edited it. – SidCool Nov 25 '10 at 7:57
Even for a long stick? – mbq♦ Nov 25 '10 at 9:27

3 Answers

up vote 12 down vote accepted

Actually the ripples are not circular at all. See photo below.

alt text

For example, a long stick will generate a straight water front on from its sides and circular waves from its edges. Something similar to a rectangle where the two short sides are replaced by semi-circles.

As the waves spread, the straight front will retain its length, whereas the circular sides will grow in bigger and bigger circles, hence the impression that on a large body of water the waves end up being circular - they are not, but very close.

The reason that an irregular object generates "circular" ripples is therefore this: as the waves propagate, the irregularities are maintained but spread across a larger and larger circular wave front.

A very good example of this phenomenon is the Cosmic Microwave Background (CMB) where electromagnetic waves from the Big Bang are measured after having spread for 13.7 billion years. Although the CMB is really, really smooth - because of the "circular ripple" effect, if you like, we can still measure small irregularities, which we think are due to the "irregular shape" of the Big Bang at a certain time.

alt text

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6  
The usual name for "as the waves propagate, the irregularities are maintained but spread across a larger and larger circular wave front" is Huygen's principle en.wikipedia.org/wiki/Huygens%27_principle – j.c. Nov 25 '10 at 15:47
Correct, I've added your link to the answer. Thank you. – Sklivvz♦ Nov 25 '10 at 19:42
up vote 2 down vote

waves always travel with a constant speed. For waves in water to travel at a constant speed they need to be circular. And hence the ripples in water are always circular.

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This has the same problem as Santosh Linkha's answer. It only explains waves coming from a point perturbation. See my comment below his answer. – Arnoques Nov 29 '11 at 22:31
up vote 1 down vote

I don't think that water ripples are circular in nature because of the size of the object is small compared to ripples because you can even create perfect ripple with kitchen knife.

It is circular in nature because the transmission of disturbance from a source event takes place in a wave fashion in a 2-D medium. And intensity of wave is distributed equally in all directions of medium.

As you know wave motion all particles in ripple execute a harmonic motion, they remain in phase i.e. particles at equal distance from center remain at equal height, there by creating a circle.

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This only explains why waves that come from a single point perturbation are circular. But for an irregular object the waves coming from different point will interfere and will create a pattern that's not circular. It will become asymptotically circular when the waves get much bigger than the original perturbation. – Arnoques Nov 29 '11 at 22:29

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