Optical path length

In optics and telecommunication, the term optical path length has the following meanings:

1. In a medium of constant refractive index, n, the product of the geometric distance and the refractive index.
2. In a medium of varying refractive index, the integral of n δs, where δs is an element of length along the path, and n is the local refractive index.

Note: Optical path length is proportional to the phase shift that a lightwave undergoes along a path.

Fermat's principle states that the path light takes between two points is the path that has the minimum optical path length.

Source: from Federal Standard 1037C and from MIL-STD-188