Mathematical Training of Lifeguards 
Rod in the water
And the same argument can be used to explain another phenomenon, represented in the following figure: the perfectly straight but apparently broken cylindrical rod when immersed in the water.
The scheme represented in the first picture of the next figure allows us to understand the apparently broken rod we see in the second picture, as well as to verify that the part that is under the water is not perceived as exactly straight, as it might seem the case in a first observation.
The following interactive application allows us to modify some parameters and to see the different behaviours that result from those changes.
