Tag Archives: optics

Eddies – surface imprint and optical properties

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?

Refraction of light in water – coins and lenses.

More on what water can do to light.

Remember my fascination with dandelions? Just to remind you:



Especially in combination with coins and water droplets, dandelions are a source of nearly endless entertainment:


1 NOK with water droplet in the hole in the middle.

See how much cooler 1 NOK coins become only by adding a little water?


The upper coin does not have water in the hole, the lower one does. See the difference?

Here the same two coins in the sun – see how the water droplet in the coin in the back focusses the light whereas the empty hole in the coin in the front is just a hole?


The coin in the foreground doesn’t have water in the hole, the one in the back does. See how they affect light differently?

Pretty cool stuff. And in the next post I’ll show you what this very effect does out in the real world!


Left coin without water, right coin with water in the hole in the middle.

Refraction of light in water.

What water can do to light.

In the last post, I showed you a couple of pictures of a vase filled with dandelions.


Dandelions and light being focussed by the water in the vase.

Turns out this might not have been enough of a clue, so here we go:

Isn’t it amazing time and time again how water refracts light and makes things look distorted?


Refraction of light in water.

This can be used for all kinds of cool experiments, provided you have the right kind of coins at hand:


1 NOK with a water droplet in the middle, acting as a lens. See how the dandelions are flipped upside down in the middle of the coin?

This kind of stuff keeps me entertained for quite some time!