On Nov 8, 1895, Wilhelm Conrad Rontgen found X-rays. He was looking into cathode rays with a vacuum tube when he saw a dim glow from a fluorescent screen close by, even though it was covered with heavy black cardboard. He figured this meant there was a fresh, invisible kind of radiation causing it, so he called them X-rays since X is what you call something you don't know in math. Because of this huge find, he got the first Nobel Prize in Physics in 1901.