X-ray

High energy electromagnetic radiation of wavelengths in the range 10 to 0.1 nanometres. The only photons which are usually more energetic are gamma rays (which originate in the nucleus of atoms). X-rays are extra-nuclear and have two sources: (1). A characteristic (e.g.K- or L-) X-ray photon is created and emitted when an electron drops down orbits to fill a vacancy in an innermost shell. The wavelength of the photon is unique to the atom and affords an unambigious method of identifying the element by use of an X-ray spectrometer . (2). A bremsstrahlung photon is produced when a high speed positive or negative electron decelerates when passing through the electric field in the close vicinity of an atomic nucleus. The spectrum in this case is continuous up to the maximum energy of the bombarding electrons.

Electromagnetic radiation of very short wavelength and very high energy; x-rays have shorter wavelengths than ultraviolet light but longer wavelengths than cosmic rays.

A very high energy form of electromagnetic radiation (though not as high energy as gamma rays ). X-rays typically have wavelengths from a few picometers up to 20 nanometers. X-rays easily penetrate soft tissue, which makes them useful in medical imaging and in radiation therapy.

The part of the electromagnetic spectrum whose radiation has somewhat greater frequencies and smaller wavelengths than those of ultraviolet radiation. Because x-rays are absorbed by the Earth's atmosphere, x-ray astronomy is performed in space.