1. 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: Reflection Total internal reflection Refraction Dispersion of light Absorption == Simple materials used == Lenses Prisms Mirrors Transparent materials (e.g. optical filters) Translucent materials (e.g. frosted glass) Opaque materials == References == from wikipedia.org