Experiments and Calculations Relative to Physical Optics
Light sent two ways overlaps into bright and dark bands — so light must be a wave.
Shine light through two openings and, where the two beams overlap, you don't get a brighter blur — you get stripes. That's the fingerprint of a wave.
The big idea
Drop two stones in a still pond and the ripples spread out and cross. Where two crests meet they pile into a bigger crest; where a crest meets a trough they flatten to nothing. Thomas Young showed that light does exactly the same thing. Send one beam of light along two slightly different paths and recombine them, and they add up in some places and cancel in others — making a row of bright and dark bands he called fringes.
Particles can't do that. Two streams of tiny bullets only ever make more bullets; they never cancel to leave a dark gap. So the stripes were proof that light travels as waves. And the spacing of the stripes told Young something no one had ever measured: the length of a single light wave — astonishingly tiny, a few ten-thousandths of a millimetre, and different for each colour.
How it came about
For a hundred years the towering authority of Isaac Newton had settled the matter: light was a hail of tiny particles, or "corpuscles." To suggest otherwise in England was almost heresy. Thomas Young was the man to do it — a physician and one of the last true polymaths, who as a sideline helped decipher the Rosetta Stone's hieroglyphs.
Around 1803 Young found a demonstration so simple it needed only sunlight. He let a thin sunbeam into a dark room through a pinhole, then held a narrow card edge-on in the beam, splitting it in two. On the wall beyond, the card's shadow was crossed by coloured stripes — and, impossibly for a simple shadow, its very centre was bright. He read this to the Royal Society in 1803. The response was scorn: an anonymous critic ridiculed him, and Britain clung to Newton for another fifteen years, until the Frenchman Augustin Fresnel gave Young's idea unanswerable mathematics.
Why it mattered
Settling that light is a wave reshaped all of physics. It led, through Fresnel and then Maxwell, to the discovery that light is an electromagnetic wave, and it gave science a precision tool — interference — for measuring tiny distances by counting fringes. Two centuries later, the very same experiment, done one particle at a time, became the clearest window into the strangeness of quantum mechanics.
A way to picture it
Think of two loudspeakers playing the exact same pure tone. Walk slowly along the wall in front of them and the sound swells and fades, swells and fades: in some spots the two sound waves arrive in step and reinforce, in others they arrive out of step and cancel into a quiet patch. Young's bright and dark fringes are those loud and quiet spots — but for light instead of sound, and so finely spaced that you see them as stripes rather than hear them as silence.
Where it sits
Christiaan Huygens had argued in 1690 that light spreads as waves, but Newton's rival particle picture had carried the day. Young's fringes tipped the balance back; Fresnel's mathematics sealed it; and Maxwell later revealed what kind of wave light is — a ripple of electric and magnetic fields. The same two-path experiment now lives on at both extremes of physics: in giant interferometers like LIGO that listen for gravitational waves, and in the quantum lab, where it shows single particles behaving like waves.
Whenever two portions of the same light arrive at the eye by different routes, either exactly or very nearly in the same direction, the light becomes most intense when the difference of the routes is any multiple of a certain length, and least intense in the intermediate state of the interfering portions; and this length is different for light of different colours.