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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.

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

• Reflection
  • Total internal reflection
• Refraction
• Dispersion of light
• Absorption

• Lenses
• Prisms
• Mirrors
• Transparent materials (e.g. optical filters)
• Translucent materials (e.g. frosted glass)
• Opaque materials