doubleslit

The double-slit experiment, also called the Young's double-slit experiment, demonstrates interference between light waves passing through two closely spaced slits. It shows the wave nature of light and, in modern contexts, the wave-particle duality of matter.

Setup: two narrow, parallel slits separated by distance d, illuminated by coherent light of wavelength λ. On a screen at distance L, the two waves travel slightly different distances, producing an intensity pattern of alternating bright and dark fringes. For small angles, bright fringes satisfy d sin θ = m λ; dark fringes satisfy d sin θ = (m+1/2) λ.

If each slit has finite width a, the single-slit diffraction envelope modulates the interference pattern; the total intensity is I ∝ cos^2(π d sin θ / λ) × sinc^2(π a sin θ / λ). The angular spacing of bright fringes is Δθ ≈ λ/d, and the linear fringe spacing on a screen a distance L away is Δy ≈ λ L/d.

Quantum interpretations: The experiment can be performed with individual photons or electrons, and still yields an interference pattern as more particles are detected; which-path information destroys interference. This illustrates quantum superposition and coherence.

Applications and variants: Demonstrates coherence, used in interferometry, calibrating optics, and as a teaching tool; variations include changing slit separation, slit width, using different wavelengths, and matter-wave versions.