Talking about how a deformation in the surface leads to light being focussed in different ways here and here, another example came to my mind. Remember how my mom and I were watching the standing waves at the Pinnau a while back? That was the same place where we also observed the “shadows” of the eddies, so as we were playing with water and light anyway, this happened:
A stick poking through the water surface. See the deformation of the surface and the effect that has on focussing the light at the ground (when you follow the stick down to the ground and then follow its shadow)?
See how the stick is deforming the water surface? This again leads to a focussing of light at the ground which you can observe if you follow the stick until you reach the ground and then follow its shadow.
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.
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?