Past exam of the physics 2 paper of the University of Oxford / 2014 resit / 10 / 1 / Solution

Fermat's principle states that the actual ray makes the travel time stationary under small variations of its path with endpoints fixed. Equivalently, for a stationary medium it makes the optical path length $\int nds$ stationary. “Stationary” allows minima, maxima, or other stationary paths; it does not require a strict minimum in every optical system.