Light and Color 
River beach
If the beach is a river beach and assistance is only provided from one of the banks, it may be useful for the lifeguard to know the best way to help a bather, who, on the other bank, feels unwell after a bath...
The following interactive application handles this case (assuming the river has parallel straight banks and no current at that time).
In particular, the reader can see that swimmers who swim, in proportion to the running speed, faster than others, must follow different courses.
This concrete example is analogous, in the case of light, to the paths of light rays that cross a plate with parallel faces.
