Richard Feynman called it the one experiment that holds "the only mystery" of quantum mechanics. Send a wave through two slits and its two halves interfere, painting bright and dark fringes on the screen. Then switch to Particles and fire them one at a time — each lands as a single random dot, yet the same fringe pattern builds up from thousands of them. Close a slit and the interference vanishes. It all runs on your device.
Barrier slit spacing is schematic; the pattern uses the real values
Particles0
Fringe spacing Δy5.50 mm
Wavelength550 nm
The pattern is built by particles that never met. Fire photons one at a time and each lands at a single random point — but thousands of them pile up into the same bright-and-dark fringes a continuous wave would make. Neighbouring bright fringes sit a distance Δy = λL/d apart (wavelength × screen distance ÷ slit separation), so redder light, a further screen, or closer slits all spread the fringes wider. Close one slit and the interference vanishes into a single blob — the clue that each particle really does pass through both slits at once.
How It Works
Two overlapping waves, a pattern of reinforcement and cancellation, and particles that reproduce it one at a time.
1
Two waves overlap
Light reaching the two slits spreads out from each and overlaps on the far side. Where the two waves arrive in step they reinforce into a bright fringe; where they arrive half a wavelength out of step they cancel into a dark one. The result is the evenly spaced interference pattern — spacing Δy = λL/d.
2
A diffraction envelope
Each slit has a finite width a, so its own light diffracts into a broad central lobe with fainter side lobes. This single-slit envelope, sinc²(π·a·sinθ/λ), modulates the brightness of the interference fringes — which is why the outer fringes fade. Widen a slit and the envelope narrows; close one slit entirely and only the envelope remains.
3
Particles build the pattern
In Particles mode each photon's landing point is drawn at random from that intensity curve. Any single dot looks random, but the probability of landing where the wave is bright is high and where it is dark is zero — so the fringes emerge statistically as the count grows. Same physics, revealed one particle at a time.
What is the double-slit experiment?
The double-slit experiment sends light (or any particle, such as electrons) through two narrow, closely spaced slits onto a screen. Instead of two bright bands, the screen shows a pattern of many alternating bright and dark fringes — an interference pattern — because the waves from the two slits add up in some places and cancel in others. It is one of the most famous demonstrations in physics.
Why does it show that light behaves as a wave?
Only waves interfere. Light passing through two slits produces evenly spaced bright and dark fringes exactly where two overlapping waves would reinforce or cancel. Close one slit and the fringes vanish, leaving a single broad diffraction blob. The reappearance of fringes when both slits are open can only be explained if light travels as a wave through both slits at once.
What happens if you send particles through one at a time?
Each particle lands at a single point on the screen, apparently at random. But as thousands accumulate, the interference fringes emerge from the scattered dots — even though the particles went through one by one and never met. This is the heart of wave–particle duality: each particle behaves like a wave passing through both slits and interfering with itself.
What is the fringe spacing formula?
The spacing between neighbouring bright fringes is Δy = λL/d, where λ is the wavelength, L is the distance from the slits to the screen, and d is the separation between the slits. Longer wavelengths, a more distant screen, or more closely spaced slits all spread the fringes further apart.