You can see “shadows” of eddies on the ground!

As everybody who has ever watched a bath tub drain knows – eddies do lead to a deformation of the water’s surface. Here is an example of what that looks like in the real world:

Eddies coming off the edge of a rock in a current.

In case you don’t see the eddies like pearls on a string coming off the edge of that rock in the picture above, watch the movie below – it’s much clearer when it is moving! Do you see the surface dipping where those little eddies are?

And in the movie below you can see how there is a shadow at the bottom underneath each of those eddies.

Why, you ask? Well, remember this from last weeks post?

Two 1 NOK coins, the one in the back with a water droplet in the hole in the middle.

The water droplet with the convex surface focusses the light. The eddies with a concave surface, on the other hand, does have the opposite effect: As the light enters the water, it is refracted away from its previous axis, leading to a “shadow” at the bottom underneath the eddy. How cool is that?