WARPLAB

What am I looking at?

What you are looking at

Green laser light goes through two narrow slits and lands on a screen 1 m away, one photon at a time. Each photon makes one dot at a random place. Thousands of dots build bright and dark bands, the fringes, and the histogram under the screen fills up to the curve the model predicts. Switch on the which-path detector and the fringes go.

Controls

On this device:

Mouse

  • Click 10, 100, 1000 or 10 000 to put that many photons on the screen at once.
  • Replay starts the build-up again: photons land one by one, then faster, up to 10 000. The step-by-step version has no Replay.
  • Fire one adds a single photon, Fire 100 adds a hundred, Clear empties the screen.
  • Drag the Slit separation slider to move the slits between 50 and 500 µm apart.
  • Click Which-path detector to switch it on or off.
  • Click the source badge beside Photon energy to see where the constants come from.

Keyboard

  • Tab moves through the photon buttons, Fire one, Fire 100, Clear, the detector and the slider.
  • Enter or Space presses the button in focus.
  • On the slider, the arrow keys move the slits 5 µm at a time; Home and End jump to 50 and 500 µm.
  • The screen and histogram carry a text description with the photon count and the fringe spacing, for screen readers.
  • H or ? opens this panel. Esc closes it.
Other devices

Touch

  • Tap 10, 100, 1000 or 10 000 to put that many photons on the screen at once.
  • Tap Replay to watch the build-up again from one photon.
  • Tap Fire one, Fire 100 or Clear to add photons by hand or start over.
  • Drag the Slit separation slider with one finger.
  • Tap Which-path detector to switch it on or off.
  • Tap the source badge beside Photon energy to see where the constants come from.

Headset and AR

  • On a VR headset, Enter VR puts you in the lab: the slits on a bench at waist height, a 2.4 m screen 3 m beyond them.
  • Point a controller and pull the trigger, or point your hand and pinch, to press a button on the panel.
  • The panel fires one, 100 or all 10 000 photons, clears the screen, moves the slits 25 µm closer or wider and switches the detector.
  • Aim at the floor and press to teleport; the right stick turns you 30° at a time.
  • In AR, aim at a table until a ring appears, then tap the screen (phone) or pull the trigger or pinch (headset) to place a 1:10 model.
  • On a phone, drag one finger to move the model and pinch two fingers to resize it. On a headset, use Place again, Smaller and Bigger on the panel.
  • The first session in VR and in AR opens a short intro that teaches these controls step by step; Controls on the panel opens it again.

Glossary

Photon
The smallest amount of light there is. Each photon lands at one point on the screen, never spread out, and where it lands is random.
Why it matters The pattern only shows once many photons have landed, so it is a pattern of chances.
Photons on the screen
How many photons have landed so far, up to 10 000. The same photons land in the same places on every visit.
Screen: photon hits
The dark band at the top. Each dot is one photon where it hit the screen. While there are under 1000, a white ring marks the newest one.
Position on screen (mm)
Distance across the screen from the point straight behind the middle of the two slits, in millimetres. The view shows 40 mm either side.
Hits
The bars under the screen: how many photons landed in each narrow strip of it. A bar full to its share meets the prediction curve.
Prediction
The solid curve: the model's chance of a photon landing at each point on the screen. It does not need any photons; the hits grow into it.
Why it matters Comparing bars to the curve is how you test the model.
Interference
What happens when two waves meet. Where they are in step they add up to bright; where one is up and the other down they cancel to dark.
Fringes
The bright and dark bands on the screen. Bright fringes are where the waves from the two slits arrive in step, dark ones where they cancel.
Fringe spacing
Distance between the centres of two neighbouring bright fringes: Δy = λL/d. At 100 µm it is 5.32 mm. With the detector on it reads No fringes.
Why it matters Move the slits closer and the fringes spread out; move them apart and they crowd together.
Slit separation
Distance d from the centre of one slit to the centre of the other, from 50 to 500 µm. A micrometre (µm) is a thousandth of a millimetre.
Slit width
How wide each slit is: 20 µm here, fixed. Each slit alone spreads light into a broad band about 27 mm either side of the middle.
Why it matters That band is the outline the fringes sit inside, so they fade towards the edges.
Wavelength
Distance λ from one crest of the light wave to the next: 532 nm, the green of a laser pointer. A nanometre is a millionth of a millimetre.
Photon energy
The energy one photon carries, E = hc/λ: 2.33 eV for 532 nm light. The constants h, c and e come from CODATA 2022; the badge shows the source.
Screen distance
Distance L from the slits to the screen: 1.0 m. A farther screen would spread the fringes further apart.
ψ₁, ψ₂
The wave amplitudes for a photon reaching a point through slit 1 or through slit 2. They can be positive or negative, so two of them can cancel.
|ψ₁ + ψ₂|²
The chance of a hit when nothing records the slit: add the two amplitudes first, then square. The cancelling gives the dark fringes.
|ψ₁|² + |ψ₂|²
The chance of a hit when the detector records the slit: square each amplitude, then add. Nothing cancels, so the light falls in one broad band.
Which-path detector
A detector that records which slit each photon took. When it is on, the fringes go, the same amount of light lands in one broad band, and the slit separation no longer matters.
Why it matters Knowing the path and seeing interference cannot both happen.
No detector
The faint dashed curve shown while the detector is on: the prediction with the detector off, kept for comparison.
Scale
In AR, the size of the model compared with the real bench. It starts at 1:10, about 28 by 40 cm, and goes from 1:40 up to 1:1.

All help and questions

Double slit

Fire photons through two slits one at a time and watch the fringes build. Switch on a which-path detector and they vanish.

Photons on the screen1

Each dot is one photon. Each one lands at a single random point, yet together they fill the histogram below up to the curve the model predicts.

Nothing records which slit a photon took, so the two paths add as waves, |ψ₁ + ψ₂|², and cancel in the dark fringes.

Fringe spacing
5.32 mm
Wavelength
532 nm
Photon energy
2.33 eV
Slit width
20 µm
Screen distance
1.0 m

Sources

Physics benchmarks

  • Intensity equals cos²(πd sinθ/λ)·sinc²(πa sinθ/λ): central maximum 1, zeros at d sinθ = (m+½)λ and a sinθ = mλ.
  • With the which-path detector the density is the incoherent sum |ψ₁|² + |ψ₂|² = ½·sinc²: the dark fringes fill in, the separation d drops out and the total light is unchanged.
  • Random single hits, from 1000 to 100 000, reproduce the analytic density in a χ² test (p > 0.001), with and without the detector; at 10 000 they rule out the other pattern.
  • Photon energy is E = hc/λ with h, c and e from CODATA 2022: 2.33 eV at 532 nm.

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