Double-slit experiment simulation
The double-slit experiment is quantum mechanics in one picture. Each particle arrives at a single point, yet the hits build up into interference fringes, as if each one passed through both slits as a wave.
Interactive, free and runs in your browser. Part of the Quantum Mechanics simulations.
What you will see
From above, a wave leaves the source, passes two slits and interferes: bright bands where the wavelets reinforce, dark nodal rays where they cancel. Particle hits appear on the detector one at a time and slowly form the fringes. Turn on which-slit measurement and the wavelets stop interfering: the nodal rays and the fringes disappear.
Try this
- Let the detector fill and compare the dot pattern with the intensity curve beside it.
- Increase the wavelength and watch the fringes spread apart.
- Switch on "Measure which slit" and watch the pattern turn into one broad hump.
Common misconception
Measurement does not "disturb the particle" in a simple mechanical way. Any process that records which slit was used removes the interference term between the two paths, so the probabilities add instead of interfering.
Exam-style question
Electrons with wavelength 50 pm pass through slits 1.0 μm apart onto a screen 2.0 m away. What is the fringe spacing?
Δy = λD/d = (50 × 10⁻¹² m × 2.0 m) / (1.0 × 10⁻⁶ m) = 1.0 × 10⁻⁴ m, or 0.1 mm.
Key equations
Δy = λD / dλ = h / p
Curriculum
AQA A-level physics, IB Physics HL C.3 and E.1.