Rontgen rayR["o]ntgen ray\ (Physics) Any of the rays produced when cathode rays strike upon surface of a solid (as the wall of the vacuum tube). R["o]ntgen rays are noted for their penetration of many 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. R["o]ntgen. They also ionize gases, but cannot be reflected, or polarized, or deflected by a magnetic field. They are regarded as nonperiodic, transverse pulses in the ether. They are used in examining opaque objects, as for locating fractures or bullets in the human body.