What is the double slit experiment?

The double slit experiment fires particles (photons, electrons, atoms) at a barrier with two narrow slits. When both slits are open and no measurement is made, the particles produce an interference pattern as if they were waves passing through both slits simultaneously. When a detector is placed to determine which slit each particle passes through, the interference pattern disappears and particles behave classically. This reveals wave-particle duality — a fundamental feature of quantum mechanics.

What is wave-particle duality?

Wave-particle duality is the principle that every quantum object (electron, photon, atom) exhibits both wave-like and particle-like properties depending on how it is measured. In the double slit experiment, particles behave as waves when unobserved (producing interference fringes) but as particles when measured (producing two bands). This was first explained by Louis de Broglie in 1924, who proposed that all matter has an associated wavelength λ = h/p, where h is Planck's constant and p is momentum.

How are interference fringes calculated?

In Young's double slit experiment, constructive interference (bright fringes) occurs where the path difference from the two slits equals an integer multiple of the wavelength: d sin θ = mλ, where d is slit separation, θ is the angle from centre, m is an integer, and λ is wavelength. Destructive interference (dark fringes) occurs where d sin θ = (m + ½)λ. The fringe spacing on a screen at distance L is approximately Δy = λL/d.

What is the observer effect in quantum mechanics?

The observer effect in quantum mechanics refers to the fact that measuring a quantum system inevitably disturbs it. In the double slit experiment, placing a detector to determine which slit each particle uses provides 'which-path information' that collapses the quantum superposition, destroying the interference pattern. This is not due to the clumsiness of measurement but a fundamental consequence of quantum entanglement between the particle and the measuring device.

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🧪 Double-Slit Experiment Simulator - Quantum Interference and Wave-Particle Duality

Simulate the famous double-slit experiment interactively. Observe quantum interference fringes, wave-particle duality, and how observation affects quantum systems. Essential for GCSE and A-level quantum physics.

Quantum Physics2DModerate60 FPS

⚙ Under the hood

This simulation models the double-slit experiment, showcasing quantum interference and wave-particle duality. Users can observe how particles behave as waves when not observed.

Double SlitQuantum Interference

2D · HTML5 Canvas 2D · 60 FPS target · runs fully client-side, no install