These were discovered accidentally by German physicist W.C. Röntgen (hence also known as Rontgen rays) in 1895, while studying cathode rays in a discharge tube. Not being quite certain about their nature then, he called them X-radiation.
In 1912 another German physicist, Max von Laue, showed that the regular atomic arrangement in crystals provides a natural grating of the right spacing to produce an interference pattern on a photographic plate when X rays pass through such a crystal. This experiment, besides identifying X rays as electromagnetic radiation, started the use of X-rays for study of the detailed atomic structure of crystals. Since the outer electrons, to which optical absorption and emission are related, are used for chemical bonding, while the energies of inner-shell electrons remain essentially unaffected by atomic bonding, the properties of elements that make up a material are more accurately determined by the emission, absorption, or fluorescence of X rays than by visible or ultraviolet light, the former being more associated to the inner-shell electrons.
The spectral composition and characteristic frequencies of X rays emitted by an X-ray source indicates the characterstic material of that source. This feature is used in optical spectroscopy wherein the X-rays emitted correspond to the differences of the internal electronic atomic and molecular energies.
X- rays have very short wavelengths of the range 10-9 to 10-11 m. They have a very high frequency and also a very high penetrating power.
Within months of their discovery, X-rays became indispensable in orthopedic and dental medicine to develop the X-ray photographs (radiographs) of the human body's interior. A contrast between body parts is produced by the different scattering and absorption of X-rays by bones and tissues. The bones, which are opaque to X-rays appear dark, whereas the soft tissues through which X-rays can easily pass appear lighter. With the use of X-rays for medical purposes constantly going under development, highly sophisticated procedures like Computerized Axial Tomography (CAT) have come into the medical world.
X-rays are produced in X-ray tubes by the deceleration of energetic electrons, by placing a metal target in their path or by accelerating electrons moving at relativistic velocities in circular orbits.
They are detected by their photochemical action in photographic emulsions or by their ability to ionize gas atoms. X-ray photons result in current pulses, whose rate can be measured to find their intensity. Instruments used for this purpose are called Geiger counters.
X-ray astronomy has revealed very strong sources of X-rays in space. In the Milky Way, the most intense sources include certain double star systems (one of the two stars being either a compact neutron star or a black hole), which may be generating X rays more than 1,000 times as intense as the total amount of light emitted by the Sun. At the moment of their explosion, supernovae also emit a huge fraction of their energy in a burst of X-rays.
X-ray images of the Sun can yield important clues to solar flares and other solar changes that can affect space weather.
X-rays can be very harmful and can easily kill living cells, if dosed in high quantities. Exposure of the body cells and tissue to large doses of X-radiations, which have a very high ionising power, can result in DNA abnormalities that may further lead to cancer and birth defects.
Nowadays, even though safer low-energy X-rays are being used often in hospitals, dentists' offices and laboratories, lead shielding is still recommended to prevent prolonged exposure to these harmful radiatons.