Path difference

In optics, optical path length (OPL) or optical distance is the product of the geometric length of the path followed by light through a given system, and the refractive index of the medium through which it propagates. In many textbooks, it is symbolically written as Λ. A difference in optical path length between two paths is often called the optical path difference (OPD). OPL and OPD are important because they determine the phase of the light and governs interference and diffraction of light as it propagates.

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