On November 8, 1895, Wilhelm Conrad Roentgen (accidentally) discovered an image cast from his cathode ray generator, projected far beyond the possible range of the cathode rays (now known as an electron beam). Further investigation showed that the rays were generated at the point of contact of the cathode ray beam on the interior of the vacuum tube. It has showed that they were not deflected by magnetic fields, and that they penetrated many kinds of matter.
After his discovery, Roentgen took an X-ray photograph of his wife's hand, which clearly revealed her wedding ring and her bones. The photograph electrified the general public and aroused great scientific interest in the new form of radiation. Roentgen named the new form of radiation X-radiation (X for "unknown"), hence the term X-ray. It's also called Roentgen rays, though this term is used only in Germany.
The images produced by X-rays are due to the different absorption rates of different tissues. Calcium in bones absorbs X-rays the most, so bones look white on a film recording of the X-ray image, called a radiograph. Fat and other soft tissues absorb less, so they look gray. Air absorb the least, so lungs look black on a radiograph.
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