The theory of optics

For centuries white light was thought to be the pure, simple thing, and colors were held to be modifications of it — white light somehow stained or weakened by passing through glass or by mixing with shadow. Newton reversed this. White light, he showed, is the mixture; the colors are the simple, irreducible ingredients. A prism does not add color to light; it fans out colors that were already there, because each color is bent (refracted) by its own fixed amount as it passes through the glass — red least, violet most. Recombine the fan with a second prism or a lens and you get white light back.

This was the first time a familiar, seemingly unanalyzable sensation — color — was pinned to a measurable physical property of light, its "refrangibility." It converted a matter of philosophy into a matter of experiment. It also had an immediate practical bite: because a simple lens bends different colors by different amounts, it cannot bring them all to one focus, so every single-lens telescope surrounds bright objects with colored haze. Newton's own theory told him this blur was unavoidable in a lens, which is why in 1668 he built the first working reflecting telescope, focusing with a curved mirror instead — a design that dominates large telescopes to this day.

Splitting light into its fixed components is the seed of spectroscopy: a century later Fraunhofer mapped the dark lines in the Sun's spectrum, and those lines turned out to be chemical fingerprints, letting us read the composition of stars we will never touch. Optics-theory also underlies lens and camera design, the science of color, and — once Herschel found in 1800 that the spectrum continues past red into invisible heat — the realization that visible light is a small slice of a far larger range. The controlled understanding of how lenses and light behave is a direct prerequisite of photography.