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
Presets
Wavelength550 nm · green
Fringe spacing5.50 mm
Visible fringes9
Slits2 · a = 0.020 mm
Photons landed0
Relative intensity across the screen. The filled curve is the two-slit pattern I = sinc²(π·a·sinθ/λ)·cos²(π·d·sinθ/λ); the dashed line is the single-slit diffraction envelope that fades the outer fringes. The bracket measures the fringe spacing Δy = λL/d between the central maximum (m = 0) and its neighbour (m = 1).
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.
Reading the simulation
What the fringes, the dashed envelope and the intensity cross-section are telling you.
1
The fringes and their spacing
Bright and dark bands stripe the screen because the two slits act as two light sources whose waves reinforce or cancel. Neighbouring bright fringes sit a fixed distance Δy = λL/d apart — the caliper on the screen and on the cross-section measures it between the central maximum (m = 0) and its neighbour (m = 1). Slide the wavelength or screen distance up, or the slit separation down, and watch the readout and the bracket widen together.
2
The envelope that dims the edges
The dashed curve is the single-slit diffraction envelope, set by the slit width a. It rides over the fringes and fades the outer ones — so only the fringes under its bright central lobe really stand out, which is the Visible fringes count. Widen a slit and the envelope narrows, hiding more fringes; narrow it and the envelope broadens. Switch to Single slit and only this envelope is left, with no fringes at all.
3
Dots that become fringes
In Particles mode each photon lands as one dot at an apparently random spot, and the faint dashed line shows the intensity curve they are drifting toward. No single dot is a fringe, yet thousands of them pile up into exactly the bright-and-dark pattern the wave predicts — the signature of wave–particle duality, each photon interfering with itself through both slits.
How It Works
A path difference between two slits, a diffraction envelope from each slit's width, and intensity read as probability.
1
Two paths, one path difference
Light reaches a point on the screen along two routes, one from each slit, and their lengths differ by d·sinθ. When that gap is a whole number of wavelengths the waves arrive in step and add to a bright fringe; when it is a half-integer they arrive opposed and cancel. The bright orders fall at d·sinθ = mλ, which for a distant screen spaces them evenly by Δy = λL/d.
2
Each slit diffracts
A slit of finite width a spreads its own light into a broad central lobe with fainter side lobes — the single-slit pattern sinc²(π·a·sinθ/λ), with its first dark ring at sinθ = λ/a. This envelope multiplies the interference fringes, so the full intensity is I = sinc²(π·a·sinθ/λ)·cos²(π·d·sinθ/λ). Close one slit and the cos² term drops out, leaving only the envelope.
3
Intensity is probability
Quantum mechanics reads that intensity curve as the probability of finding a photon at each spot. The simulator draws each landing at random from it (rejection sampling), so dots crowd where the curve is high and never fall where it is zero. Fire enough and the statistics rebuild the smooth pattern — the same physics whether you watch a continuous wave or count photons one by one.
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. With this lab's defaults — λ = 550 nm, L = 1.0 m and d = 0.10 mm — that works out to Δy = 5.50 mm, the figure shown in the readout.
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Classroom activity
A ready-to-assign lab using the simulation above. Free to use — nothing to install or sign up for, and no student data leaves the browser.
High school, College10–20 minInteractive simulation
Learning objectives
Explain interference of waves passing through two slits.
Relate fringe spacing to wavelength, slit separation, and screen distance.
Connect the pattern to the wave nature of light.
Variables to change
Wavelength
Slit separation
Screen distance
Procedure
Set a mid-range wavelength and note the bright-fringe spacing on the screen.
Increase the wavelength and observe how the fringe spacing changes.
Increase the slit separation and observe the fringe spacing again.
Observations
Record the bright-fringe spacing as you vary wavelength and slit separation.
Questions
What causes the bright and dark fringes?
How does increasing the wavelength affect fringe spacing?
How does increasing the slit separation affect fringe spacing?
Write the condition for bright fringes.
What does this experiment demonstrate about the nature of light?
Explanation
Light from two slits overlaps and interferes. Where crests meet crests the screen is bright; where crests meet troughs it is dark. Fringe spacing grows with wavelength and shrinks with slit separation, per d·sinθ = mλ — direct evidence of light’s wave nature.
Answer key (for instructors)
1. Constructive interference (path difference = whole number of wavelengths) makes bright fringes; destructive interference (half-wavelength differences) makes dark fringes.
2. Fringe spacing increases — spacing is proportional to wavelength.
3. Fringe spacing decreases — spacing is inversely proportional to slit separation.
4. d·sinθ = mλ, where m = 0, 1, 2, … and d is the slit separation.
5. That light behaves as a wave; the interference pattern cannot be explained by particles alone.
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