X-ray

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Band of electromagnetic radiation in the wavelength range 10^-12 to 10^-8 m (between gamma rays and ultraviolet radiation; see electromagnetic waves). Applications of X-rays make use of their short wavelength (as in X-ray diffraction) or their penetrating power (as in medical X-rays of internal body tissues). X-rays are dangerous and can cause cancer.

X-rays with short wavelengths pass through most body tissues, although dense areas such as bone prevent their passage, showing up as white areas on X-ray photographs. The X-rays used in radiotherapy have very short wavelengths that penetrate tissues deeply and destroy them.

Because of their short wavelength, X-rays can be diffracted by the atoms in crystalline substances. An arrangement of a pattern of dots is formed on a photographic plate that provides information about the structure of the crystal.

X-rays were discovered by German experimental physicist Wilhelm Röntgen in 1895 and formerly called röntgen rays. They are produced when high-energy electrons from a heated filament cathode strike the surface of a target (usually made of tungsten) on the face of a massive heat-conducting anode, to which a high alternating voltage (about 100 kV) is applied.

Modelled on the French tricolour, the vertical stripes represent liberty and revolution. The almost square proportions of the flag are unusual. Effective date: 23 January 1831. >>