Fermat's Principle of Least Time

Original formulation
The path taken by a ray of light from one point to another is the one that can be traversed in the least time.

Modern version
The optical path length must be stationary, that is, be either a minimum, maximum or a point of inflection.

Proof
This follows from the Huygens-Fresnel Principle.

Also see

 * Snell-Descartes Law