Optical path refers to the trajectory that a light ray follows as it propagates through an optical medium. It is the length of the path that light travels through a medium and the refractive index of that medium. Optical path length (OPL) is a fundamental concept in the field of optics, and it is used to determine the phase of light and governs interference and diffraction of light as it propagates. The geometrical optical-path length or simply geometrical path length (GPD) is the length of a segment in a given optical path, i.e., the Euclidean distance integrated along a ray between any two points. 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.

Factors affecting the optical path include the path of light in the medium or between two media, the materials used, such as lenses, prisms, mirrors, transparent materials (e.g., optical filters), translucent materials (e.g., frosted glass), and opaque materials.

In summary, optical path refers to the path that light follows as it propagates through an optical medium, and optical path length is the length of the path that light travels through a medium and the refractive index of that medium.