Young's Double Slit

Newton's corpuscles had held for a century, and Thomas Young's demonstration to the Royal Society in 1803 was the thing they could not survive. Light through two narrow openings does not make two bright patches. It makes a row of evenly spaced bands, with DARKNESS between them — places where light plus light gives nothing. Particles cannot do that. Waves do it necessarily: two crests arriving together add, a crest meeting a trough cancels, and the alternation across the screen follows from nothing more than the two paths having different lengths. What makes it an instrument rather than an argument is the arithmetic. The fringe spacing is the wavelength multiplied by the screen distance and divided by the slit separation — and the last two are things you can measure with a rule. Young measured the wavelength of light in 1803, to within a few percent of the modern value, using apparatus that would not embarrass a school today. Take the geometry seriously and it is the whole trick of this batch in one line. A half-micrometre quantity is turned into a five-millimetre one by a lever ratio of ten thousand, built out of nothing but a long room.