συμβολή: a throwing together. In Greek physics, interference.
Young's double slit: the experiment, every major variation and what it might mean. Every fringe pattern on this site is computed from the wave equations, not drawn by hand.
In 1803 Thomas Young showed the Royal Society that light added to light can produce darkness. A century later the same experiment, run one particle at a time, became the cleanest demonstration of quantum mechanics. Feynman called it the only mystery, a phenomenon “impossible, absolutely impossible, to explain in any classical way.” Two centuries on, physicists are still running new versions of it: deciding after the fact whether to look, erasing the record of which slit was used, cutting the slits in time instead of space, and pushing molecules of twenty-five thousand atomic mass units through the apparatus to see where (if anywhere) the quantum world gives out.
This bench walks the whole arc. The demos are honest: one wave-optics engine (a Huygens phasor sum with exact path lengths) drives every pattern you see, and a test suite checks it against the closed-form results.
Thomas Young, 1773–1829: physician, physicist, decipherer of demotic and the man who bet against Newton on the nature of light.
Waves, slits and fringes: a live ripple view and a real-scale screen pattern, all under your control.
Send particles through singly and watch the fringes assemble dot by dot, then vanish the moment you ask which slit each one used.
Complementarity made quantitative: dial the detector strength and watch V² + D² pin to 1.
Wheeler's delayed choice, the eraser and the sorting demo that shows why nothing rewrites the past.
The 2023 time-domain double slit: two openings in when, interference in colour.
Electron to cricket ball at true de Broglie wavelengths, plus the decoherence dial that explains who falls off the ladder.
Copenhagen, Many-Worlds, real computed Bohmian trajectories and Feynman's curling arrows. Same maths, different stories.
A laser pointer, a strand of hair, two polarisers: the experiment on your kitchen table.
Thomas Young was born in Milverton, Somerset, in 1773, the eldest of ten children in a Quaker family, and was reading fluently at the age of two. By his mid-teens he had worked through Latin, Greek, French and Italian and begun on Hebrew, Chaldean, Syriac, Samaritan, Arabic, Persian, Turkish and Amharic. It is a list that sounds like a joke until you notice what he later did with it. He trained as a physician in London, Edinburgh and Göttingen, and was elected a Fellow of the Royal Society at twenty-one for explaining how the eye focuses: the lens changes shape. His biographer's title has stuck as the standard one-line summary: the last man who knew everything.
It matters to this story that Young came to light through the eye. He measured astigmatism first (his own), proposed in 1802 that colour vision rests on just three receptor types (the trichromatic theory, confirmed a century and a half later) and thought harder than anyone alive about what vibrations the eye actually receives. Sound was known to be a wave; Young's wager was that light is too. The trouble was that the rival theory (light as a stream of tiny particles, or "corpuscles") belonged to Newton, and in England around 1800 disagreeing with Newton was close to a moral failing.
Young's decisive idea, the one this whole site is named for, was the principle of interference (1801): when two portions of light arrive by different routes, their effects add, and if one route is half a wavelength longer than the other, the addition is a cancellation. Light plus light can make darkness. He showed the Royal Society supporting cases first (the colours of thin films, the fringes inside shadows), and in his 1803 Bakerian Lecture described splitting a sunbeam with a slip of card one-thirtieth of an inch wide and watching fringes appear in the shadow, fringes that vanished when he blocked the light on one side of the card. The canonical two-slit arrangement (one light source behind two pinholes, stripes on a screen) appears in his Course of Lectures on Natural Philosophy of 1807, with the wavelengths of red and violet light calculated correctly to within a few percent, from measurements made with sunlight and card.
“The middle of the shadow was always white… one edge of the card being covered, the fringes disappeared.”
The reception was brutal. Henry Brougham (lawyer, later Lord Chancellor) savaged the wave papers anonymously in the Edinburgh Review, dismissing work now taught to every physics undergraduate as containing nothing worth the Society's attention. Young wrote a pamphlet in reply. It sold one copy. He largely withdrew from optics and went back to medicine; it took Fresnel's mathematics and Arago's experiments in France, a decade and more later, to win the argument Young had already made in English.
He was, in short, exactly the sort of person you would want to have designed physics' most famous experiment: someone incapable of staying inside one discipline, who took an idea from water and sound and had the nerve to point it at Newton. The experiment outgrew him (the chapters that follow take it places Young could not have imagined), but the principle at its heart is his, and it has never once failed.
Young, A Course of Lectures on Natural Philosophy and the Mechanical Arts (1807) · Young, Bakerian Lecture, Phil. Trans. (1804) · Robinson, The Last Man Who Knew Everything (2006) · Feynman, Lectures on Physics vol. III ch. 1 · Tonomura et al., Am. J. Phys. 57, 117 (1989) · Kim et al., PRL 84, 1 (2000) · Tirole et al., Nature Physics 19, 999 (2023) · Fein et al., Nature Physics 15, 1242 (2019).