Double-Slit Interference Simulator
Coherent point source, two slits, far-field detection screen. Switch between the Huygens wavefront view and the single-particle accumulation view, the same buildup pattern documented in electron-interference experiments.
USER GUIDE
Controls
- Wavelength sets the source color and directly sets fringe spacing (larger lambda spreads fringes apart).
- Slit separation (d) is the center-to-center distance between the two slits. Smaller d spreads fringes apart.
- Slit width (a) controls the width of the single-slit envelope that limits how many fringes are visible before intensity drops off.
- Screen distance (L) is the distance from the slits to the detection screen. Only valid in the far-field (Fraunhofer) regime this model assumes.
- Arrival rate controls how many particles register per second in the accumulation view. It does not change the physics, only playback speed.
Two views, one experiment
Wavefront view shows the classical Huygens-Fresnel picture: circular wavefronts spreading from each slit and interfering. This is the wave-optics explanation of the pattern.
Accumulation view shows what is actually recorded on a real detector: individual, discrete particle strikes arriving one at a time and only gradually building into the same interference pattern. This is the picture confirmed by single-electron buildup experiments (see Methodology).
METHODOLOGY AND REFERENCES
Intensity model
The simulator computes screen intensity from the standard Fraunhofer two-slit diffraction formula, combining a single-slit diffraction envelope with two-slit interference:
where theta is the angle from the central axis, a is slit width, d is slit separation, and lambda is wavelength. Fringe spacing on the screen follows:
Particle accumulation method
Each detected particle's landing position is drawn by rejection sampling from I(theta) treated as a probability density. This reproduces, statistically, the buildup behavior reported for single-electron double-slit experiments: individual, random-looking strikes that only reveal fringes once enough have accumulated. The simulator does not compute a literal quantum wavefunction evolution; it samples the same probability distribution that quantum theory predicts and that has been experimentally measured.
Compute path
When available, sampling and acceptance testing run in a WebGPU compute shader (WGSL), evaluating many candidate particles in parallel per frame. If WebGPU is unavailable in the visitor's browser, the simulator falls back to an equivalent single-threaded JavaScript sampler at reduced throughput. Both paths use the identical formula above, so results are numerically consistent; only speed differs.
Validation
The "measured" fringe spacing readout is computed directly from the accumulated histogram of detected positions (peak-to-peak spacing of the strongest bins) and compared live against the theoretical delta_y. Convergence between the two values as particle count grows is the model's built-in self-check.
References
- T. Young, "Experiments and Calculations Relative to Physical Optics," Philosophical Transactions of the Royal Society, 1804. Original double-slit interference experiment.
- A. Tonomura, J. Endo, T. Matsuda, T. Kawasaki, H. Ezawa, "Demonstration of single-electron buildup of an interference pattern," American Journal of Physics 57, 117 (1989). Basis for the accumulation view.
- R. Feynman, R. Leighton, M. Sands, The Feynman Lectures on Physics, Vol. III, Ch. 1. Conceptual treatment of single-particle interference.
- E. Hecht, Optics, 5th ed., Pearson. Standard reference for the Fraunhofer diffraction formula used here.
PROVENANCE
What this tool is
An educational visualization of a well-established, repeatedly verified physical result. The intensity formula and fringe-spacing relation are textbook optics, unchanged since Fraunhofer diffraction theory was established in the nineteenth century. The single-particle accumulation behavior is drawn from peer-reviewed experimental literature, cited above.
What this tool is not
It is not a first-principles quantum field solver, not a measurement device, and not a substitute for laboratory data. All parameter ranges (wavelength, slit geometry, screen distance) are chosen to stay inside the paraxial, far-field regime where the formula above is valid. Pushing sliders to extreme ratios of slit width to wavelength can produce visually plausible but physically untested output; treat values far outside typical visible-light laboratory setups with caution.
Version
Model formula and reference set last reviewed by Astrophyzix against the sources listed in Methodology. No proprietary or unpublished data is used anywhere in this tool.
GOVERNANCE AND LIMITATIONS
Approximations in force
- Fraunhofer (far-field) and paraxial (small-angle) approximations throughout.
- Fully coherent, monochromatic, scalar-wave treatment. No polarization, no partial coherence, no source bandwidth.
- Particle accumulation is statistical sampling of the known probability distribution, not a dynamical quantum simulation.
- No relativistic, gravitational, or medium (refractive index) effects are modeled.
Intended use
Classroom and self-study demonstration of interference and the wave-particle behavior confirmed by the cited experiments. Not intended for research-grade metrology or as a citation source in place of the primary literature above.
Rights and redistribution
Copyright 2026 Astrophyzix. All rights reserved. Redistribution of this tool, in whole or in part, in source or compiled form, is not permitted without prior written permission from Astrophyzix.