<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> $\int n\,ds$ stationary. “Stationary” allows minima, maxima, or other stationary paths; it does not require a strict minimum in every optical system.

Solution

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

 Solution (source code)