length of the path that would be travelled at the speed of light in vacuum in a duration equal to the phase delay of a given electromagnetic wave between two given points
Note 1 to entry: Between two points in the same ray path, the optical path length equals the line integral of the refractive index along the ray path:
where n is the refractive index of the medium at each point in the direction of the path and ds the curve element along the path.
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