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Fermat's Principle

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