The optical path length along a ray is the integral of its refractive index over geometric length. A difference in optical path length produces a phase difference , where is the vacuum wavelength.
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Optical path length (OPL) is a concept used in optics to describe the effective distance that light travels through a medium, taking into account both the physical distance and the refractive index of the medium. It is defined as the product of the distance that light travels through a medium and the refractive index of that medium.