x–ray

In November, 1895, the German scientist Wilhelm Röntgen was conducting an experiment on the properties of electron tubes. He noticed that a fluorescent surface in the vicinity of the tube would light up even if shielded from the tube's direct light. A thick metal object would block some of the rays, while a less dense object such as wood would cast only a weak shadow. Röntgen's explanation was that the tube produced some type of invisible radiation that could pass through substances that blocked ordinary light. Because he did not know the nature of this radiation, he named it X-Strahl—translated into English as X-ray—based on the mathematical use of x to indicate an unknown quantity.