Home / Optics & Photonics / Wave Optics / Double-Slit Interference Fringe Spacing

Formula & Calculator

Double-Slit Interference Fringe Spacing

In a double-slit experiment, the bright fringes on the screen are evenly spaced. This formula gives the distance between adjacent bright fringes. It equals the wavelength of light multiplied by the distance from the slits to the screen, divided by the slit separation. For example, red light (longer wavelength) produces wider fringe spacing than blue light. This is used in optics labs to measure wavelengths and in understanding interference phenomena.

Written by the Vorzep Team

Optics Wave Optics Interference

⟹ Solve⟷Δy, λ, L, d

Fringe Spacing (Δy)

Wavelength (λ)

Screen Distance (L)

Slit Separation (d)

Please fix the errors above.

Solve for: Fringe Spacing (Δy) Wavelength (λ) Screen Distance (L) Slit Separation (d)

Presets:

Fringe Spacing—

—

Δy: —λ: —L: —d: —

✓ Copied!

Fringe Spacing Gauge—

Small (< 0.001 m) Medium (0.001–0.01 m) Large (> 0.01 m)

Δy = λ·L / d · For small angles, the fringe spacing is uniform in the double‑slit interference pattern.

Variables

Symbol	Quantity	Unit
Δy	Fringe spacing	m
λ	Wavelength of light	m
L	Distance to screen	m
d	Slit separation	m

What it means

The fringe spacing in Young’s double‑slit experiment is constant and given by the formula. It is used to measure the wavelength of light and to calibrate optical systems. The formula is derived from the interference condition. Understanding this is essential for wave optics and for interpreting interference patterns. It also applies to grating interference. The spacing increases with wavelength and distance to the screen and decreases with slit separation.

Worked example

Double‑Slit Fringe Spacing – Two Detailed Examples

Real‑World

Scenario: In a double‑slit experiment with slit separation d = 0.5 mm, a laser of wavelength 632.8 nm is used, and the screen is L = 2 m away. The fringe spacing Δy = λL/d = 632.8×10⁻⁹ × 2 / (0.5×10⁻³) = 2.531×10⁻³ m = 2.531 mm. The student uses this to measure the wavelength of the laser by measuring the fringe spacing, providing a hands‑on verification of the wave nature of light.

Parameter	Value
λ (nm)	632.8
L (m)	2
d (mm)	0.5

1Δy = 632.8e-9 × 2 / (0.5e-3) = 2.531e-3 m = 2.531 mm

Result 2.531 mm ✓ Fringe spacing

Scenario: A physics lab uses a green laser (λ = 532 nm) with d = 0.3 mm and screen distance L = 1.5 m. The fringe spacing is Δy = 532×10⁻⁹ × 1.5 / (0.3×10⁻³) = 2.66×10⁻³ m = 2.66 mm. Students measure this spacing to determine the wavelength and understand the interference pattern. The experiment demonstrates the direct relationship between wavelength and fringe separation.

Parameter	Value
λ	532
L	1.5
d	0.3

1Δy = 532e-9 × 1.5 / (0.3e-3) = 2.66e-3 m = 2.66 mm

Result 2.66 mm ✓ Green laser spacing

Insight: The fringe spacing in double‑slit interference is directly proportional to wavelength and screen distance, and inversely proportional to slit separation. This allows wavelength measurement using a known slit geometry.

Common mistakes

Double‑slit fringe spacing: Δy = λ·L / d – the distance between adjacent bright (or dark) fringes.
λ: Wavelength of light – in metres.
L: Distance from slits to screen – in metres.
d: Slit separation – in metres.
Units: Δy in metres – consistent with L and d.
Assumes: Small angles (sinθ ≈ tanθ).

Applications

Double‑slit interference fringe spacing, Δy = λ·L/d, gives the distance between adjacent bright (or dark) fringes in Young's experiment, where λ is wavelength, L is distance to screen, and d is slit separation. This formula is used in wave optics to measure wavelength, to characterise light sources, and to design interference‑based sensors. Engineers use it in optical metrology for displacement and surface profiling. By measuring fringe spacing, they can determine the wavelength of light or the slit separation. This equation is also used in holography and in testing optical components. Understanding interference fringe spacing is essential for many precision measurement applications.

Wavelength measurement using Young's experiment
Optical metrology for displacement and strain
Interferometer design and calibration
Testing optical surfaces and coatings
Education on wave interference

Related calculators · Wave Optics

Suggest an improvement

Found a mistake? Have a better example? Let us know.
You may send up to 3 suggestions per 24 hours.

Page details

1,865

Views

Published July 13, 2026
Last updated October 11, 2026