Optical path

The path that a light ray follows as it passes through an optical medium or system is often called the optical path. The physical length of an optical device can be reduced to less than the length of the optical path by using folded optics. The optical path length as defined in optics is the length of the path multiplied by the refractive index of the medium.
Factors affecting optical path.
Path of light in medium, or between two media is affected by the following:
Simple materials used.
‘change in optical length is equal to wavelength of light passed’