On 8 November 1895, working on cathode rays in his laboratory in Würzburg, the German physicist Wilhelm Conrad Röntgen (1845–1923) saw something unexpected. Although he had covered the tube completely in a cardboard sheath, a fluorescent screen a few metres away glowed in the dark. Neither visible light nor the known cathode rays could pass through that cardboard. Röntgen realised an unknown, new ray existed and named it the "X-ray," after the letter mathematics uses for the unknown.

Over the following weeks he studied the ray obsessively. It passed through paper, wood, even thin metal; but dense materials — bone, lead — stopped it. His most striking experiment was with his wife Anna Bertha: she placed her hand between the ray and a photographic plate. In the resulting image the flesh was a faint shadow while the bones — and her ring — appeared clearly. Looking at the picture, she is said to have exclaimed, "I have seen my death." For the first time humanity was looking inside a living body without opening it.

The spread of the discovery was extraordinarily fast. Röntgen published his finding in December 1895; within months hospitals around the world were imaging broken bones, lodged bullets, and swallowed objects with X-rays. Medicine could, for the first time, map the inside of the body in real time and harmlessly (as was then thought). Röntgen received the first Nobel Prize in Physics in 1901 and refused to patent his discovery, leaving it open to all.

X-rays did not only transform medicine; they opened a new window on science. A few months later, investigating X-rays, Becquerel found radioactivity; then atomic physics used these rays to solve the structure of crystals and of DNA. Making the invisible visible became one of the most fertile methods of twentieth-century science.