Compute optical path length and optical path difference for interference analysis.
由折射率与几何路程，按 光程 = n·L 计算光在介质中的等效真空路径。
| Item | Formula / Note | Source / Standard |
|---|---|---|
| Definition of optical path length | OPL = n · d, the product of refractive index and geometric distance | ISO 80000-7:2019, Quantities and units - Part 7: Light and radiation |
| Fermat principle | Light follows the path for which the optical path length is stationary, usually minimal | Fermat P. (1662); Born M, Wolf E, Principles of Optics (7th ed.), Cambridge |
| Relation to phase | Phase difference Δphi = (2·pi/lambda_0) · OPL, where lambda_0 is the vacuum wavelength | Born M, Wolf E, Principles of Optics (7th ed.), Cambridge Univ. Press |
| Special case in vacuum | In vacuum n = 1, so the optical path equals the geometric distance | Born M, Wolf E, Principles of Optics (7th ed.) |