Optical Path Length
Also known as: OPL · Optical Distance
Inside glass light crawls slower, so a 1 mm slab packs in more wave crests than 1 mm of vacuum. Multiplying the real thickness d by the refractive index n converts physical distance into 'how much phase the wave accumulated' — that is what interference cares about.
A beam crosses a slab of variable index and thickness; wave crests bunch up inside, and the accumulated optical path length n*d is tallied.
Equivalent forms
Multiply a real distance by the index and you get the distance light experiences — geometry and phase fused into a single product.
Unit systems
- SI:
- d, OPL in m; n dimensionless
- CGS:
- d, OPL in cm
- Imperial:
- d, OPL in in
Where it holds
Fermat's principle recast optics in terms of the path that minimizes travel time — equivalently, the path of stationary optical path length n*d. The OPL became the central quantity unifying ray and wave optics.
Why does a thin sheet of glass shift an interference pattern as if the light traveled extra distance?
Light slows in a medium, so it fits more wave cycles into the same physical gap. The optical path length n*d counts those cycles — it is the distance light 'feels', and it governs every interference and phase calculation.