‖ Young’s Experiment
Interference of waves from two slits: a simulation with a screen, an intensity graph, single photons and a fringe calculator.
Slit separation d60 px
Wavelength λ14 px
Slit width a8 px
View:
Mode:
Examples:
What the screen shows
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Fringe spacing Δy ≈ λL/d
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Bright fringes on the screen
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First envelope zero (from the center)
Interference
Pattern on the screen
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Photons on the screen
Maxima: d·sin θ = m·λ Minima: d·sin θ = (m + ½)·λ
Intensity: I(θ) = I₀ · cos²(π d sin θ / λ) · sinc²(π a sin θ / λ)
Fringe spacing: Δy ≈ λ·L / d
Intensity: I(θ) = I₀ · cos²(π d sin θ / λ) · sinc²(π a sin θ / λ)
Fringe spacing: Δy ≈ λ·L / d
- The “Rays” view is a simplified geometric diagram: rays go from the source through the slits to the screen and pack more densely into the bright fringes. It is an illustration, not particle trajectories. The “Wave field” view shows the real sum of the waves from the slits: a plane wave on the left, and overlapping circular waves behind the barrier.
- On the right are the screen and the intensity graph I(y). The solid line shows the current pattern, the dashed line the alternative (with or without the detector).
- Narrow the slits and the central intensity hump widens: this is single-slit diffraction, which acts as an envelope for the fringes.
- In the “One photon at a time” mode each particle lands on the screen at random, and the fringes emerge only from many hits. Switch on the detector and you will see the fringes vanish.
- Color corresponds to wavelength; the pixels in the simulation are arbitrary (λ from 8 to 30 px ≈ 400–700 nm).
Enter the real parameters of the experiment: the wavelength, the slit separation, the distance to the screen and the slit width. You will get the positions of the fringes on the screen.
Wavelength λ, nm
Slit separation d, mm
Distance to the screen L, m
Slit width a, mm
Maximum order m
Examples:
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Fringe spacing Δy ≈ λL/d, mm
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Position of the mth maximum from the center, mm
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Angle θm, °
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Highest order m
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First envelope zero λL/a, mm
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Fringes in the central lobe
Maximum m: ym = L · tan θm, sin θm = m·λ / d
Envelope zero: sin θ = λ / a
Fringes in the central lobe: 2·⌈d / a⌉ − 1
Envelope zero: sin θ = λ / a
Fringes in the central lobe: 2·⌈d / a⌉ − 1
Fringes for different colors (d = 0.25 mm, L = 2 m)
| Color | λ, nm | Δy, mm | θ₁, ° |
|---|---|---|---|
| Violet | 400 | 3.2 | 0.09 |
| Blue | 450 | 3.6 | 0.1 |
| Green | 532 | 4.26 | 0.12 |
| Yellow | 580 | 4.64 | 0.13 |
| Red | 650 | 5.2 | 0.15 |
| Dark red | 700 | 5.6 | 0.16 |
Young’s Experiment Online: Double-Slit Simulation and Calculator
A free interactive tool that shows how waves from two slits overlap and form bright and dark fringes on a screen. Change the slit separation, the wavelength and the slit width, and watch the wave field and the intensity graph. The “One photon at a time” mode reproduces how the pattern builds up from individual hits, and the detector at the slits shows why which-path information destroys interference. A separate calculator computes the positions of the maxima for real lasers and lamps.
Key concepts
Interference
Superposition of coherent waves: where crests coincide, the wave is reinforced; where a crest meets a trough, it weakens or vanishes.
d·sin θ = m·λ → maximum
Single-slit diffraction
A wave spreads out behind a narrow opening. The narrower the slit, the wider the intensity envelope under which the interference fringes lie.
First zero: sin θ = λ / a
Coherence
A constant phase difference between the waves from the two slits. Without it the fringes blur out. A laser is coherent; an ordinary lamp only partially.
Path detector → fringes vanish
Where it is used
- School and university wave-optics laboratories
- Measuring a laser’s wavelength from the fringe spacing
- Interferometers, including gravitational-wave detectors
- Diffraction of electrons and neutrons by crystals and slits
- Explaining wave–particle duality and measurement in quantum mechanics
- Anti-reflective coatings and thin films (the colors of a soap bubble)
How to use
- In the “Waves” mode, move the d slider and watch how the fringe spacing changes
- Change the wavelength λ: the color and the width of the fringes change together
- Narrow the slit a to see the diffraction envelope
- Switch to “One photon at a time” and watch the fringes emerge from dots
- Switch on the detector at the slits, and the interference disappears
- In the “Calculator” tab, enter the real λ, d, L, a of your experiment
Frequently asked questions
What is Young’s double-slit experiment?
It is an experiment in which a coherent wave (light, water, sound, or even electrons or photons) passes through two narrow slits and hits a screen. Instead of two bright spots, the screen shows alternating bright and dark fringes — an interference pattern. In this way Thomas Young demonstrated the wave nature of light in the early 19th century.
Why do fringes appear on the screen?
Each slit emits its own wave. At points where the path difference equals a whole number of wavelengths (d·sinθ = mλ), the waves reinforce each other — a maximum appears. Where the path difference equals a half-integer number (d·sinθ = (m + ½)λ), the waves cancel each other — a minimum appears.
What does the fringe spacing depend on?
For small angles the spacing between neighboring fringes is Δy ≈ λL/d. It grows with the wavelength λ and the distance to the screen L, and decreases when the slits are moved apart (d increases). That is why red light gives wider fringes than blue.
Why do two slits give fringes even when photons arrive one at a time?
Each photon hits the screen at a random point, but the distribution of many such hits reproduces the interference pattern. The probability of landing at a point is determined by the sum of the amplitudes from the two slits. In the “One photon at a time” mode you can see the fringes gradually emerge from individual dots.
What happens if you find out which slit the particle passed through?
Interference disappears when which-path information exists: instead of fringes, two overlapping spots form on the screen, and the pattern is the sum of two separate patterns, one from each slit. It is important that what matters is the possibility of learning the path (for example, a detector at a slit), not the consciousness of an observer. Switch on the detector option in the simulation to see the difference.