Today we know that x-rays are electromagnetic waves having a very short wavelength (shorter than a few tenths of an ångström); they are of atomic origin, like light, but are generated by electronic transitions in the innermost levels of atoms having a high atomic weight and excited, for example, by bombardment with electrons having sufficient energy. They are produced especially when the beam of electrons in a cathodic ray tube is made to fall on a metal plaque placed in front of the cathode (anticathode).
In all cases of bombardment of a material of high atomic weight with accelerated electrons, together with the x-rays produced by electronic transitions with well-defined frequencies, which are characteristic of the material of which the target is made, there are also x-rays of all the frequencies below a maximum that depends only on the energy of the electrons.
This continuous part of the x-ray spectrum, known as bremsstrahlung, is caused by the fact that an electron passing near an atomic nucleus of the target is brusquely slowed down and can irradiate, in a single quantum, all of its kinetic energy or only a part of it; thus, according to Planck's law, in the first case the frequency will be maximum and in the second case smaller. By accelerating tensions that range from a few tens to a few hundred thousands of volts, as in ordinary x-ray tubes, this second mechanism (nuclear braking) is responsible for only a small percentage of the x-rays produced. ...continues