Seaside physics teaching: Looking at water to understand optics

Guest post by Mirjam

Since I am currently on vacation, I asked my dear friend Mirjam for help with this weeks experimental friday. And I can tell you, you’re in for a treat. She is deeply into oceanography and especially wavewatching, which, as you can probably tell, go very well together. And what makes her thoughts and ideas so outstanding is how she manages to see waves and patterns every time she looks upon water. I truly admire that. So let’s see what she can teach us about optics by looking at water.

One thing that makes optics a difficult and unintuitive topic for many people is that all the things discussed in optics – wave lengths, wave fronts, dispersion, interference, refraction, and many more – are very hard to observe, at least when talking about light waves. Demonstrations can be done using other kinds of waves, too, but a quick google search shows that ripple tanks (the textbook demonstration to explain optics) cost typically around 1,000€.

But of course there are other, cheaper demonstrations that can help make optics more easy to grasp and more relevant. A couple of #ExperimentalFridays ago, for example, Alice wrote a post on using water drops to explain optics, in which she presented a nice and easy experiment. Today, I want to show you a different, completely hassle-free way of using water to explain optics, and that is using puddles, lakes, the ocean, any water surface you have available, as your ripple tank.

Let’s for example consider the situation below, the river Schlei in northern Germany, where we can observe several interesting things at once.

On the right side of the wave breaker (in the right half of the picture), wave crests are straight. That’s where the wind has had a wide and long area over which it has been blowing over the water, a long fetch, creating these waves and driving them down the river. At least until they can’t continue on, because there suddenly is a wave breaker in their way.

But if you look in the foreground of the picture on the left, there are waves downwind of the wave breaker, in its lee, filling the space behind it. These waves are not straight anymore and driven by the wind. They are curved and diffracted from the straight waves.

In a physics textbook, this situation would be shown for “diffraction at a single slit”, and look like it’s depicted on the left of the picture below: Straight wave fronts arrive at a narrow slit and then, on the other side of the slit, curved wave fronts start propagating as if from a point source.

On the right, the textbook figure is modified to fit the situation we saw in the photo: The slit is now very wide (almost the width of the river), therefore we see parts of the diffracted wave field (the curved waves in the lee of the wave breaker) that continue on as straight wave crests that pass through the wide “slit” undisturbed further offshore.

Now, back to the original picture. Keep in mind the waves that propagate to the left, out of the picture, look like that where they hit the shore:

If we were to turn our head to the left and look a little downstream, this is what we would see:

Here we have the same wave crests that were propagating out of the previous frame.

But what we also see is a second set of wave crests at an angle to the first one. Do you notice how in the spots where wave crests hit wave crests, the waves are especially high? That’s interference, another thing that we learn about in optics.

And the reason why we see two different wave crests at an angle to each other is that waves are reflected on the shore of the river. In a textbook this would look like it’s shown below: the angle between the incoming ray and the “mirror” is the same as the one to the outgoing ray.

Adjusted to the situation I showed in the photograph, the textbook picture would look like it’s shown below on the right: Several waves are reflected at the same time.

Isn’t it awesome how we can visualize optics, just by looking at water? It’s as easy as this to make optics teaching fun. It’s actually a fun activity that you can do anytime and anywhere, and it is called #wavewatching. Will you do it next time you look at water?

Of course, in many cases the situations you find on lakes or the sea is more complicated than what I showed here. If you see something really awesome but can’t quite figure out what is going on there (or did figure it out but want to see if I can figure it out, too), please feel free to contact me and send me a picture of your waves and I will try to explain it. I call that #friendlywaves, and here are a lot of examples of where I’ve done that. And if you enjoy #wavewatching in general, feel free to follow my blog over at mirjamglessmer.com/blog

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