Double slit experiment simulator

Send light through two narrow slits and a pattern of bright and dark fringes appears on the screen. Send it one photon at a time and the same pattern still emerges, dot by dot.

What you see

Each photon lands at a single point, so individual hits look random. After thousands of photons the hits pile up into fringes. The wave-like pattern belongs to the ensemble, not to any one photon.

The physics

For slits separated by d and a screen at distance L, bright fringes are spaced by roughly Δy = λL/d for small angles. The intensity follows I ∝ cos²(πd sinθ / λ), modulated by a single-slit envelope from the finite slit width. Smaller wavelength or wider slit separation gives tighter fringes.

Which-path information

If the setup lets you tell which slit each photon used, the fringes vanish. Interference needs the two paths to stay indistinguishable. That is the core lesson of the experiment.

Try it

Open the simulator, change the wavelength and slit separation, and watch how the fringe spacing moves. Then compare single-photon buildup with the continuous-light view.