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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.

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