What is the fringe spacing formula?

Fringe spacing (β) = λL / d, where λ is wavelength, L is the screen distance, and d is the slit separation. It tells you the distance between consecutive bright (or dark) fringes.

What is path difference?

Path difference is the difference in distance each wave travels from its slit to a point on the screen. Constructive interference (bright fringe) occurs when path difference = mλ; destructive interference (dark fringe) when path difference = (m + ½)λ.

Why does a shorter wavelength produce narrower fringes?

Because fringe spacing β = λL/d is directly proportional to wavelength. Violet light (~400 nm) produces tighter fringes than red light (~700 nm) under the same geometry.

What happens if I increase slit separation?

Wider slit separation (larger d) gives smaller fringe spacing, as β = λL/d. The fringes crowd together. Conversely, narrower slits spread the fringes further apart.

Does this assume a far-field (Fraunhofer) approximation?

Yes. The formula β = λL/d is valid when L >> d (screen much farther than slit separation), which is the Fraunhofer diffraction regime standard for Young's experiment problems.

Double Slit Interference Calculator

Name: Double Slit Interference Calculator
Availability: InStock
Author: Nham Vu

Enter wavelength, slit separation, and screen distance to compute bright and dark fringe positions, fringe spacing, and path differences for Young's experiment.

Experiment Parameters

Wavelength (λ) nm

Visible light: 380–700 nm

Slit Separation (d) mm

Screen Distance (L) m

Show fringes ±5 orders

Fringe Positions & Path Differences

Bright Dark

Order (m)	Type	Position y (mm)	Path diff Δ (nm)

Enter parameters above and click Calculate.

Summary

Enter wavelength, slit separation, and screen distance to compute bright and dark fringe positions, fringe spacing, and path differences for Young's experiment.

How it works

Enter the light wavelength in nanometers (e.g. 550 nm for green light).
Enter the slit separation — the distance between the two slits in millimeters.
Enter the screen distance — how far the screen is from the slits in meters.
Click Calculate to see fringe spacing, fringe positions, and path differences.
Adjust any input and recalculate to explore how each parameter affects the pattern.

Use cases

Solve Young's double-slit experiment problems in physics courses.
Verify laboratory measurements of fringe spacing against theoretical values.
Explore how changing wavelength shifts fringe positions.
Compare bright and dark fringe locations on the screen.
Understand wave-particle duality demonstrations in quantum mechanics.
Design optical setups that require controlled interference patterns.

Frequently Asked Questions

Last updated: 2026-06-10 · Reviewed by Nham Vu