twoslit

Two-slit, commonly referred to as the double-slit experiment, describes experiments in which coherent waves pass through two closely spaced slits and produce an interference pattern on a distant screen. The setup was introduced by Thomas Young in 1801 to demonstrate the wave nature of light, and since then the idea has been extended to other matter waves.

Principle: Each slit acts as a secondary source. The waves from the two slits interfere, producing regions of constructive and destructive interference depending on the path difference. In the far field the typical intensity is I(θ) ∝ cos^2(π d sin θ / λ), where d is slit separation and λ is wavelength, and for small angles Δy ≈ λ L / d gives the fringe spacing on a screen at distance L. A finite slit width w adds an overall diffraction envelope.

Quantum interpretation: When particles such as photons, electrons, or atoms pass the two slits, they show interference patterns consistent with a superposition of paths. If an experiment records which slit a particle traversed, the interference is diminished or destroyed, revealing particle-like behavior. Quantum eraser variants show that erasing the which-path information can restore interference, underscoring the role of information in quantum measurements.

Significance and applications: The two-slit arrangement is foundational in discussions of wave-particle duality and quantum coherence. It informs educational demonstrations, precision metrology, and modern experiments with single quanta and large molecules. Variants and related setups include the Mach-Zehnder interferometer and matter-wave interference experiments.