1. 1 Overview
Fermat’s principle, or the principle of least time, named after French mathematician Pierre de Fermat, is the principle that the path taken between two points by a ray of light is the path that can be traversed in the least time.
##2 Solve with Calculus
2. 3 Solve with Mechanics
The main idea: Nature will make the system in some state that has least energy.
So we can just find out the point where the forces balanced
