German physicist Wilhelm Conrad Roentgen (1845–1923) discovered X-rays in 1895—but did not understand at first what they were—which is how they got their name: In science and math, X refers to an unknown. By the end of the decade, hospitals had put X-rays to use, taking pictures (called radiographs) of bones and internal organs and tissues to help diagnose illnesses and injuries. Using the new technology, doctors could “see” the insides of a patient. In 1901 Roentgen received the first Nobel prize in physics for his discovery of a short-wave ray.