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