optical illusion, interesting physical test

[Arnaud]

Here a small video found on a scientific website https://www.youtube.com/watch?feature=player_embedded&v=G303o8pJzls .
Look how the water changes the direction of the arrow

And here a quick test made with Keyshot .... 10/10 

Rhino file available if you want to play ....

[Arnaud]

and the BIP file