Question

How is path difference related to phase difference?

Answer 1

The difference between the phases of two periodic signals is called the Phase Difference. At values of when the difference is zero, the two signals are said to be in phase, otherwise they are out of phase with each other...

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 index of refraction 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...