Dispersion in water waves

By Marcus Wilson 01/03/2013


 I’ve been perusing YouTube looking for good videos illustrating wave packets – which are bunches of waves containing different wavelengths. I want to come up with a good illustration for a second year physics paper on introductory quantum theory. This contains a lot of ‘wave’ things.

Here’s a nice one I’ve stumbled on. It shows dispersion in water waves and gives you a bit of a physics tutorial as well.  The key thing is that waves of different wavelengths (the distance from the peak of one wave to the peak of the next) move at different speeds. That means, when you throw a stone in a pond, the waves spread out. Some reach shore very quickly – these are long wavelength waves – some take much longer – the shorter wavelength ones. So, while the initial splash is always a bit of mess, wait a few seconds for the longer wavelengths to get ahead of the shorter ones, and things look rather tidier. 

In a larger-scale example, this is why ocean waves coming onshore have a very uniform nature to them – the gap between each successive wave hitting the beach is about the same if you watch over the course of only a few minutes. These waves have typically travelled a long way from a storm, and they have separated out into particular wavelengths. 

Next time you happen to throw something into a pond, watch the waves. Or watch the waves hitting a beach after a storm.