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X-rays, or roentgen rays, are electromagnetic waves in which periodically variable electric and magnetic fields are perpendicular to each other and to the direction of propagation. Thus they are identical in nature with visible light and all the other types of radiation that constitute the electromagnetic spectrum. In general, x-rays are generated as the result of energy transitions of atomic electrons caused by the bombardment of a material of high atomic weight by high-energy electrons. See Electromagnetic radiation

Following W. R. Röntgen's discovery of “a new kind of ray” in 1895, other scientists found the essential experimental conditions to prove that x-rays can be polarized, diffracted by crystals, refracted in prisms and in crystals, reflected by mirrors, and diffracted by ruled gratings. See X-ray optics

The range of x-rays in the electromagnetic spectrum, as excited in x-ray tubes by the bombardment of anode targets by cathode electrons under a high accelerating potential, overlaps the ultraviolet range on the order of 100 nanometers on the long-wavelength side, and the shortest-wavelength limit moves downward as voltages increase. An accelerating potential of 109 volts, now readily generated, produces a wavelength of 10-15 m (10-6 nm). An average wavelength used in research is 0.1 nm, or about 1/6000 the wavelength of yellow light. See X-ray tube

In diffraction, refraction, polarization, and interference phenomena, x-rays, together with all other related radiations, appear to act as waves. In other phenomena—such as the appearance of sharp spectral lines, a definite short-wavelength limit of the continuous “white” spectrum, the shift in wavelength of x-rays scattered by electrons in atoms (Compton effect), and the photoelectric effect—the energy seems to be propagated and transferred in quanta, called photons. See Compton effect, Electron diffraction, Neutron diffraction, Photoemission, Quantum mechanics

Important uses have been found for x-rays in many fields of scientific endeavor, for example, roentgen spectrometry and roentgen diffractometry. Extensive tables of the wavelengths of x-ray emission lines in series (K, L, M, and so on) and so-called absorption edges, characteristic of the chemical elements, afford the necessary information for chemical analyses, exactly as in the case of optical emission spectra and for derivation of theories of atomic structure to account for the origin of spectra. See X-ray crystallography, X-ray diffraction, X-ray powder methods


High-energy electromagnetic radiation lying between gamma rays and ultraviolet radiation in the electromagnetic spectrum. The XUV region bridges the gap between the X-ray and ultraviolet bands. X-rays, unlike light and radio waves, are usually considered in terms of photon energy, h ν, where ν is the frequency of the radiation and h is the Planck constant. X-ray energies range from about 100 electronvolts (eV) up to about 100 000 eV, corresponding to a wavelength range of about 12 nanometers (nm) to about 0.012 nm. Low-energy X-rays are sometimes called soft X-rays to distinguish them from high-energy or hard X-rays. In astronomy, thermal X-rays are produced from very high temperature gas (˜106–108 K), with nonthermal X-rays arising from the interaction of high-energy electrons with a magnetic field (synchrotron emission) or with low-energy photons (inverse Compton emission – see Compton scattering). See also nonthermal emission; thermal emission.


[′eks ‚rāz]
A penetrating electromagnetic radiation, usually generated by accelerating electrons to high velocity and suddenly stopping them by collision with a solid body, or by inner-shell transitions of atoms with atomic number greater than 10; their wavelengths range from about 10-5 angstrom to 103 angstroms, the average wavelength used in research being about 1 angstrom. Also known as roentgen rays; x-radiation.