The optical path
along a given trajectory
C between points A and B is given by:
L(AB) = | n ds
where n denotes the index of refraction, which can vary along curve C, and ds is an infinitely small section of curve C. The optical path is the equivalent distance traveled by light on curve C, as for the number of phase cycles, when compared to propagation in vacuum, which has a unitary index of refraction. The actual trajectory (trajectories) taken by light between points A and B has (have) a stationary optical path, by the Fermat principle.