X-ray

X-radiation (composed of X-rays) is a form of electromagnetic radiation. Most X-rays have a wavelength ranging from 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz (3×10¹⁶ Hz to 3×10¹⁹ Hz) and energies in the range 100 eV to 100 keV. X-ray wavelengths are shorter than those of UV rays and typically longer than those of gamma rays. In many languages, X-radiation is referred to with terms meaning Röntgen radiation, after Wilhelm Röntgen, who is usually credited as its discoverer, and who had named it X-radiation to signify an unknown type of radiation. Spelling of X-ray(s) in the English language includes the variants x-ray(s), xray(s), and X ray(s).

X-rays with photon energies above 5–10 keV (below 0.2–0.1 nm wavelength) are called hard X-rays, while those with lower energy are called soft X-rays. Due to their penetrating ability, hard X-rays are widely used to image the inside of objects, e.g., in medical radiography and airport security. As a result, the term X-ray is metonymically used to refer to a radiographic image produced using this method, in addition to the method itself. Since the wavelengths of hard X-rays are similar to the size of atoms they are also useful for determining crystal structures by X-ray crystallography. By contrast, soft X-rays are easily absorbed in air; the attenuation length of 600 eV (~2 nm) X-rays in water is less than 1 micrometer.

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X-ray (chess)

In chess, the term X-ray or X-ray attack is sometimes used (1) as a synonym for skewer. The term is also sometimes used to refer to a tactic where a piece either (2) indirectly attacks an enemy piece through another piece or pieces or (3) defends a friendly piece through an enemy piece.

The second usage is seen in the position at left, which arises from the Black Knights' Tango opening after 1.d4 Nf6 2.c4 Nc6 3.Nf3 e6 4.a3 d6 5.Nc3 g6!? 6.e4 Bg7 7.Be2 0-0 8.0-0 Re8 9.Be3 e5 10.d5 Nd4! Authors Richard Palliser and Georgi Orlov, in their respective books on that opening, both note that Black's rook on e8 "X-rays" White's e-pawn through Black's own pawn on e5. If 11.Nxd4 exd4 12.Bxd4 Nxe4 13.Nxe4 Rxe4. The identical position is reached, except that White has not played a2-a3, in the King's Indian Defense after 1.d4 Nf6 2.c4 g6 3.Nc3 Bg7 4.e4 d6 5.Nf3 0-0 6. Be2 e5 7.0-0 Nc6 8.Be3 Re8 9.d5 Nd4!

Of the position at right, arising from the Sveshnikov Variation of the Sicilian Defense, Atanas Kolev and Trajko Nedev observe, "On f1 the king is X-rayed by the f8-rook". They analyze the possible continuation 22...f5 23.exf5 Bxf5 24.Nxf5 Rxf5 25.Qg4 Bg5 (exploiting the pin along the f-file) 26.Kg2? Bxf4 27.Nxf4 Rg5 28.Nxg6+ Kg7 and White resigned in Delchev-Kotanjian, Kusadasi 2006.

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