By Zencalcy Editorial TeamLast reviewed About us
A single narrow slit spreads light into a broad central band flanked by weaker fringes. The dark bands occur where light from one edge of the slit cancels light from the other, and their positions depend on the ratio of wavelength to slit width.
Position of the first dark fringe behind a slit.
a sin θ = m λ
a is the slit width, θ the angle to the m-th minimum and λ the wavelength. The central maximum spans twice the first-minimum angle.
| Symbol | Meaning |
|---|---|
| a | Width of the slit. |
| θ | Angle to the dark fringe. |
| m | Order of the minimum, a positive integer. |
| λ | Wavelength. |
sin θ = 5.5 × 10⁻⁷ / 10⁻⁴ = 0.0055, so θ = 0.315°. The central bright band is about 0.63° wide.
Continue with the Snell's Law Calculator, Telescope Magnification Calculator, or the Thin Film Interference Calculator — or browse more optics and light tools and all physics calculators.
The first minima lie symmetrically at ±θ, so the central bright region spans from −θ to +θ, twice the spacing to one side.
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