Wilhelm Röntgen was born in Prussia in 1845, moved to the Netherlands as a toddler, and spent 40 years stateless after losing one citizenship without gaining another. As a teenager in Utrecht he was expelled for a mocking caricature of a teacher drawn by another student he refused to name, which cost him the diploma Dutch universities required. He found a back door at the Federal Polytechnic School in Zurich, which admitted him on a brutal entrance exam, and earned a mechanical engineering degree first. There he married Anna Bertha Ludwig, six years older and from a family that ran a café, and his wealthy family cut him off.

In November 1895 in Würzburg, working alone with a Crookes tube wrapped in black cardboard, he noticed a barium platinocyanide screen glowing across the dark room. He named the unknown rays X, lived in his lab for weeks testing them, and saw the bones of his own hand. A photograph of Anna’s hand, wedding ring and all, horrified her as a glimpse of her own death. He refused to patent the discovery, accepted the first Nobel Prize in physics in 1901 without giving the customary lecture, and donated the prize money to his university, only to lose his savings to postwar German inflation before dying in 1923.

• How braking radiation, Bremsstrahlung, produces X-rays when fast electrons slam to a stop
• Why light atoms in flesh let X-rays pass while calcium in bone casts a shadow
• The rapid chain from his rays to Henri Becquerel’s radioactivity and the work of Marie and Pierre Curie
• His will ordering most of his personal and scientific letters burned
• Element 111, roentgenium, and World Radiography Day as his lasting honors