Optical path length

optical path length (OPL) 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)


The refractive index, n, of a medium is defined as the ratio of the velocity, c, of a wave phenomenon such as light or sound in a reference medium to the phase velocity, v_p in the medium itself: