X - rays are electromagnetic waves of about 0,0001 nanometer wavelength. They originate because of the charged particles (e.g. the beam of electrons - cathode rays) being braked by matter. X - rays can be also a result of electrons in atom crossing energy levels (from higher to lower ones). X - radiation is characterised by a strong radiation hardness (penetrating power); the rays of smaller length are stronger penetrating.

X- rays like all the other kinds of radiation can undergo absorption, diffusion, diffraction, interference, polarisation, refraction and reflection. They cause the blackening of the photographic emulsion (the optical density), the ionisation of gas, the fluorescence of some substances, the photochemical reactions and the changes in the cells of the living organisms. The X-radiation is invisible. It is used in medicine, in studying the chemical constitution of different substances, in studying the structure of crystals. The X - radiation was discovered by Wilhelm Roentgen in 1895.