In
optics,
optical path length (OPL) or
optical distance is the product of the geometric length of the path light follows through the system, and the
index of refraction of the
medium through which it propagates. A difference in optical path length between two paths is often called the
optical path difference (OPD). Optical path length is important because it determines the
phase of the light and governs
interference and
diffraction of light as it propagates.