Standing waves

I like to see physics in strange places. Last week I took my kids to the local swimming pool and was mucking around in the kiddie pool with my youngest (he's 2). His mind was occupied with throwing a toy clown fish (he calls it Meemo, and I have to try and fight the urge to say "Found him"every time he throws it!) and the retrieving it again. So I tried a little experiment, and this one surprised me!

I talk about standing waves almost everyday to students that come through the Kickstart lab. It is such an important concept in Physics that I think I can spend a few minutes on what they are and how to make investigate them.

A standing wave is essentially 2 waves. both that interact with each other to create constructive and deconstructive interference. The result is a wave that seems not to move. The features and characteristics of a standing wave are different to that of a normal wave. A normal wave has a peak and trough, a speed you can measure by measuring it's distance travelled over time etc. A standing wave does not have these features. It has a node which is the bit in the middle that doesn't move, and the peaks and trough (they are the same thing) are called anti-nodes.

To create a standing wave, all you need to do is reflect a wave of a rigid surface (like the wall in this case) that is the same wave in the opposite direction.

Simple.

The cool bit was when I tried this in a swimming pool. You can make a wave in the pool with your hand, just move your hand through the water (close to the surface I guess) and you've got your wave. If you do the same with your other hand in the opposite direction you get interference.

This interference is the standing wave, and this is cool enough! but it gets cooler! I was mucking around with tis in the pool and looked up to see my son still throwing Nemo about and I caught in my peripheral vision the waves that I had created were still going long outside of where my hands created them, but they were not standing waves anymore. The brilliant thing about this is that I could see that my hands were creating separate waves that travelled along in their respective directions, it was only when they were interacting (between my hands) that they created a standing wave.

A very simple illustration of the complex physics of standing waves, that any one can do, and probably has done before. I love it, and next week when we go back to the pool, I'll be repeating my experiment while my son throws fish.