By Zencalcy Editorial TeamLast reviewed About us
Two nearby slits turn a single wavefront into two coherent sources. Where their waves arrive in step they reinforce; where they arrive out of step they cancel, producing evenly spaced bright and dark bands on the screen.
Fringe spacing from wavelength, slit separation and screen distance.
Δy = λ L / d
Δy is fringe spacing, λ the wavelength, L the slit-to-screen distance and d the slit separation. The small-angle approximation assumes Δy is much smaller than L.
| Symbol | Meaning |
|---|---|
| Δy | Spacing between adjacent bright fringes. |
| λ | Wavelength of the light. |
| L | Distance from the slits to the screen. |
| d | Separation between the two slits. |
Δy = 550 × 10⁻⁹ × 2 / 0.0005 = 2.2 mm. Moving the screen twice as far doubles the fringe spacing.
Continue with the Field of View Calculator, Hyperfocal Distance Calculator, or the Illuminance Calculator — or browse more optics and light tools and all physics calculators.
A larger d makes the path difference change faster across the screen, so the condition for constructive interference repeats over a smaller distance.
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.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
Free calculator · No signup
Free calculator · No signup
Free calculator · No signup
Free calculator · No signup
Free calculator · No signup
Free calculator · No signup
Content updated: September 2026