Fermat's Principle: Light follows the path of least time. The law of reflection can be derived from this principle as follows:
The pathlength L from A to B is
Since the speed is constant, theminimum time path is simply the minimum distance path. This may be found by setting the derivative of L with respect to x equal to zero.
This derivation makes use of the calculus of maximum-minimum determination, the derivative of a square root, and the definitions of the triangle trig functions.