Double Slit Experiment Calculator
Young's double-slit calculator: fringe spacing Δy = λL/d, the angle and position of any bright or dark fringe, the highest visible order and a table of fringe positions on the screen.
How it works
bright fringes: d sin θ = m λ · dark fringes: d sin θ = (m + ½) λ · position on the screen y = L tan θ ≈ m λ L ÷ d · fringe spacing Δy = λ L ÷ d · highest order m_max = ⌊d ÷ λ⌋
In Young's double-slit experiment (1801), light from two narrow slits overlaps and interferes: where the path difference is a whole number of wavelengths the waves add up (bright fringes), and where it is a half-odd number they cancel (dark fringes). For small angles the fringes are evenly spaced by λL/d, which is how the experiment measures wavelength. This calculator uses the exact angle for each fringe and the small-angle formula for the spacing; the overall brightness is also shaped by the single-slit diffraction envelope, which depends on the slit width.
Worked example
Green light of 550 nm passes two slits 0.25 mm apart and lands on a screen 1 m away. Δy = 550 × 10⁻⁹ × 1 ÷ 0.00025 = 0.0022 m, so the bright fringes are 2.200 mm apart. The first-order bright fringe is at an angle of 0.1261° and 2.200 mm from the centre, and the first dark fringe is 1.100 mm from the centre.
Sources: HyperPhysics — Young's double slit · OpenStax University Physics — 3.1 Young's double-slit interference
Frequently asked questions
What is the formula for fringe spacing?
Δy = λL/d, where λ is the wavelength, L the distance to the screen and d the slit separation. It holds when the fringes are close to the centre (small angles).
How do I find the wavelength from a double-slit experiment?
Measure the distance across several fringes and divide to get Δy, then λ = Δy·d/L. For example, 2.2 mm fringes with d = 0.25 mm and L = 1 m give λ = 550 nm.
What happens if the slits are moved closer together?
The fringes spread out: halving d doubles the spacing. Moving the screen further away or using a longer wavelength also spreads them.
What is the difference between a double slit and a diffraction grating?
A grating has thousands of slits and uses the same condition d sin θ = mλ, but its bright lines are far narrower and sharper, which makes it better for spectroscopy.
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This calculator is for educational purposes. Double-check important results before acting on them.