Refraction of light in moving water — why stuff seems to be jumping around

I was waking along Kiel fjord one morning and noticed a stone “jump” on the ground as waves went over it (and actually, that observation was the motivation to dive into stuff from the last post, too).

I think the stone only looked so curious because the rest of the ground was uniformly sandy and hence didn’t seem to move.


So seeing that jumping stone made me want to draw the optical path, which I’ve animated for you here:


Funny. I think in physics class in school, I would absolutely have hated it had I gotten the task to draw all those different diagrams, and here I really enjoyed it. Maybe because of that jumping stone? Would the right motivation have helped me as a kid to get interested in this? I think it wasn’t that I was not interested in physics, but it would never have occurred to me to sit down on my own to sketch optical paths or anything like that. Now if I could figure out what changed for me, maybe we could use that to make other people interested in physics, too?

Refraction of light in water — looking at a couple of examples

Looking at how light gets refracted when it enters water is always fascinating. There are a dozen blog posts on the topic on this blog alone, but let me talk about it again today.

In a 1908 article, Charles Judd (as summarised in Barnett & Ceci, 2002) describes an experiment where kids throw darts at a target submerged under water. Half of the kids, in addition to practicing throwing darts, are taught about refraction of light in water. While all kids do equally well on the practice task, the kids that understand the physics do a lot better when the water depth was changed. Why?


When the water depth changes, the target appears to be located in a different position than before. With shallower water, the target we see is a lot closer to the real location of the target. So kids that did not understand why they had to aim at a position off the target they saw to actually hit the target had a much harder time adjusting the way they aimed than those kids who actually understood what had changed.

But refraction is always cool to look at, even without throwing stuff. Here a picture from one of my very first blog posts (still in my house in Norway).

2013-10-24 12.13.01
“broken spoon”

Or from this blog post — a fountain in Sheffield:

2016-07-15 11.37.10

Or a swimming pool in Lüneburg that appears a lot shallower than it actually is (from this blog post).


Or a table that gets completely deformed when seen through a glass of water (from this post).

Refraction of light in water.

Is it only me or do other people sometimes also draw optical paths just for fun? ;-)


Erosion happens not only to sandy paths or beaches on the large scale, but also on a small scale to really sturdy structures like the groynes in Hastings, where each of the massive pillars has its own little dip around it, being bitten into the concrete little by little by the force of the waves, moving little stones around…


We saw it in another post already:


Yes, that’s why there are puddles exactly in the spots where one would like to sit, sheltered from the wind, the back resting against one of those pillars… ;-)

The building of sand banks

The eroding force of moving water can be seen in so many places when you pay attention. For example in a park where I sometimes go for walks, the really well-maintained paths are forever eroded and washed away by the heavy rains we’ve had recently.


In the picture below you see a green pipe opening into the pond, and what you can’t see is a second, larger pipe just to the right of the first one. Both pipes drain water from the park’s paths. Water then flows through the pond and eventually into Kiel fjord. And what happens is that all the pebbles and sand from the park’s paths end up in the drainage system and get washed into that little pond, where they get deposited in a sand bank.


Interestingly enough, water exiting the pipes seems to typically do so at such a high velocity that all the debris doesn’t get deposited right then and there, but carried downstream until the water has slowed enough that a sandbank can form. And on the sandbank you can see that larger rocks get deposited first while smaller ones are carried further with the current before they settle.

One glance — do you know which of the bottles is empty?

The other day I was sitting in my conservatory with a friend when I had to take the photo below:

Can you see how one bottle refracts light and the other one does not? What does this tell us about whether there is water in either of those bottles? I met most “normal” people wouldn’t even notice a difference.

I know, I’m a nerd, but I have so much fun “discovering” small stuff like that! :-)

One of the most exciting things about work travel?

One of the most exciting things about work travel? Staying in tons of different hotels, which all have different opportunities to play with water.

For example at a recent team event, there was this tap with a really efficient aerator, that made the hydraulic jump look even more exciting than usual:


And then at a conference last week, this happened:


Can you see what happened? Obviously, I turned the water on, and the right side of the armature fogged up because of all the cold water going through! (Even though I can assure you: My shower was nice and warm!)

And I am not even going to apologise for how excited I get by observing these kinds of things. Remember the kind of tap I have at home?

2015-11-18 13.15.31

Still the coolest tap I have ever seen! :-)