By Zencalcy Editorial TeamLast reviewed About us
No real source emits a single perfect wavelength. The spread of wavelengths sets how far light can travel while staying in step with itself, which limits the path difference an interferometer can use.
Distance over which light remains coherent.
L_c = λ² / Δλ t_c = L_c / c
L_c is coherence length, λ the central wavelength, Δλ the bandwidth and t_c the coherence time. Wavelengths must be in the same unit; the ratio makes the result scale correctly.
| Symbol | Meaning |
|---|---|
| L_c | Coherence length of the source. |
| λ | Central wavelength. |
| Δλ | Spectral bandwidth. |
| t_c | Coherence time. |
L_c = 550² / 0.1 = 3.025 × 10⁶ nm, or 3.025 mm. The coherence time is about 10 ps.
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Each wavelength produces its own fringe pattern. Once the path difference is large enough, the patterns shift out of step and the fringes wash out.
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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Content updated: September 2026