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. [ see Röntgen. ] (Physics) An X-ray; originally, the term was applied to any of the rays produced when cathode rays strike upon surface of a solid (as the wall of the vacuum tube), but now it refers specifically to electromagnetic radiation having wavelengths from 10-3 nm to 10 nm, immediately below ultraviolet radiation on the wavelength scale. Röntgen rays are noted for their penetration of opaque substances, as wood and flesh, their action on photographic plates, and their fluorescent effects. They were called X rays by their discoverer, W. K. Röntgen. They are one of the forms of ionizing radiation, which can have damaging effects on living cells. They also ionize gases, but cannot be reflected, or polarized, or deflected by a magnetic field. They are used in examining opaque objects, especially in medicine for visualizing organs and other objects inside the human body, as for locating fractures or bullets, and examining internal organs for abnormalities. [ Webster 1913 Suppl. +PJC ]