Length realization by light time and interferometry
With calibrated light frequency f, vacuum wavelength is λ₀=c/f.
Key knowledge
With calibrated light frequency f, vacuum wavelength is λ₀=c/f. In an ideal uniform-index round-trip interferometer, mirror displacement Δz=λ₀Δφ/(4πn); a pulse round-trip delay instead gives Δz=cΔt/(2n_g). Phase index n and group index n_g serve different measurement models.
Values and equations
λ₀=c/f; Δz=λ₀Δφ/(4πn); Δz=cΔt/(2n_g)
- λ₀,Δz
- Vacuum wavelength and displacement m
- c
- Exact vacuum light speed m s⁻¹
- f
- Calibrated light frequency Hz
- Δφ
- Unwrapped phase change rad
- n,n_g
- Phase/group refractive indices
- Δt
- Pulse round-trip delay difference s
Conditions and scope
- Known double-pass geometry, fixed reference path, homogeneous refractive index and an unwrapped phase/fringe count or correctly identified pulse delay.
- Frequency calibration, air temperature/pressure/composition, alignment, path stability and phase interpolation contribute uncertainty; vacuum c being exact does not eliminate them. Pulse broadening can invalidate a simple group-delay model.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
SI metre: mise en pratique (2026): Sections 2, 3a and 3b, equations 2–8 and pages 2–6; Annex 2 lists interferometric uncertainty influences. Corrects textual PDF extraction of λ/2 in equation 7 using equation 6.
2026-10-10 · Independent AI editorial review: Independent root AI editorial review of all29 authored Measurement claims/conditions, displayed equations and37 origin relations. The original26 claims were read against41 primary source sections; all5 requested corrections and the3 affected source bindings were subsequently rechecked. Three actual scalar-nonlinear/vector/dynamic applications were read, independently differentiated or integrated, and checked against the specific additional source material described below. This is reference-content review, not human specialist certification, instrument certification or experimental replication. Full-field sign-off waits for the combined final conceptual matrix. Receipt: measurement-independent-content-review.1.
Entry details
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- measurement-optical-length-realization
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