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Double-Slit Interference

Explore bright and dark interference fringes.

Young’s double slit

Coherent monochromatic light
Double-slit apparatus and interference fringesLight travels from a source through two narrow slits. Circular wavefronts overlap before reaching a screen with bright and dark bands.LIGHT SOURCEDOUBLE SLITSCREENschematic · not to scale
The marker on the screen follows the measurement control.λ=532 nm\lambda = 532\,\mathrm{nm}

Relative light intensity, II0\frac{I}{I_0}

Predicted interference intensity across the screenThe graph shows repeating bright maxima and dark minima over screen positions from minus twenty to plus twenty millimeters. The selected measurement is at 0.0 millimeters.0.00.51.0-20-1001020
Position on screen, y (mm)\text{Position on screen, } y\,(\mathrm{mm})
For an ideal pair of slits, relative intensity is the cosine squared of pi times screen position divided by fringe spacing.

Why do the bands appear?

Δr=mλ\Delta r = m\lambda

Bright fringes

The waves arrive in step and reinforce each other. The central bright fringe is order m=0m = 0.

Δr=(m+12)λ\Delta r = (m + \frac{1}{2})\lambda

Dark fringes

A half-wavelength path difference brings a crest together with a trough, cancelling the light.

Δy≈λLd\Delta y \approx \frac{\lambda L}{d}

Small-angle model

The graph assumes narrow slits, a distant screen, and small angles. It shows interference without a single-slit envelope.