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.
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 optical path.
Path of light in medium, or between two media is affected by the following:
References.
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