Double-Slit Fringe Spacing

\Delta y = \frac{\lambda L}{d}

Δy =

λ =

L =

d =

Enter your known values, leave one input blank, and solves for the missing one. Try different units for next level excitement!

Learning zone

Young's 1801 double-slit experiment settled a century-long argument by showing light interfering like water waves — and this small-angle formula is its quantitative heart. Green light (λ = 500 nm) through slits 0.25 mm apart throws fringes 2 mm apart on a screen 1 m away: a macroscopic, ruler-measurable pattern generated by a half-micron wavelength. Measure Δy, L, and d and you have measured the wavelength of light with desktop equipment.

Double-Slit Fringe Spacing

\Delta y = \frac{\lambda L}{d}

Where

$\Delta y$ = Fringe spacing
$\lambda$ = Wavelength
$L$ = Slit-to-screen distance
$d$ = Slit separation

Missing one of these? Work it out first, then come back

Fringe spacing — Diffraction Grating Equation, Speed, Distance & Time
Wavelength — Wave Speed (v = fλ), Diffraction Grating Equation
Slit-to-screen distance — Speed, Distance & Time, Inverse-Square Law for Sound
Slit separation — Force Between Parallel Wires, Gravitational Potential Energy (Orbital)

Part of this formula set

Interference and diffraction

4 formulasWhere light waves reinforce and cancel — two slits, many slits, and the thin films that colour soap bubbles and oil.