The figure shows the optical path of a Mirau-objective respectively a Mirau-interferometer. Reference beam (4 to 6) and object beam (7) have identical optical path length and can thus cause white light interference.
Optical path (OP) is the trajectory that a light ray follows as it propagates through an optical medium.
The geometrical optical-path length or simply geometrical path length (GPD) is the length of a segment in a given OP, i.e., the Euclidean distance integrated along a ray between any two points.[1]
The mechanical length of an optical device can be reduced to less than the GPD by using folded optics.
The optical path length in a homogeneous medium is the GPD multiplied by the refractive index of the medium.