By Zencalcy Editorial TeamLast reviewed About us
Light travelling through glass accumulates more phase than light covering the same geometric distance in air. Optical path length multiplies the physical distance by the refractive index, giving the distance the light would have covered in vacuum in the same time.
Effective optical distance through a medium.
OPL = n d
OPL is optical path length, n the refractive index and d the physical distance. A phase difference is 2π times the OPL difference divided by the wavelength.
| Symbol | Meaning |
|---|---|
| OPL | Optical path length through the medium. |
| n | Refractive index of the medium. |
| d | Physical thickness or distance. |
OPL = 1.5 × 1 = 1.5 m. Light in the glass is 0.5 m of optical path ahead of light that travelled the same physical metre in vacuum.
Continue with the Prism Deviation Calculator, Refractive Index Calculator, or the Single Slit Diffraction Calculator — or browse more optics and light tools and all physics calculators.
Interference depends on phase, and phase accumulates with optical rather than geometric distance. A lens works by introducing a controlled OPL profile.
Results are computed from the standard equations of classical mechanics. Unit conversions use the SI definitions published by NIST. Air resistance is ignored unless the calculator names a drag model.
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Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026