Hand with Rings

The study of cathode rays in the physics laboratory at the University of Wurzburg led to another major discovery, that of X-rays, made in 1895 by Wilhelm Conrad Rontgen (1845-1923). Rontgen observed that when cathode rays hit a metal, radiation of unknown nature was emitted (hence the name X-rays). The emission of these rays, invisible to the eye, was revealed by the fact that they caused luminescence in certain substances. Subsequent observations allowed Rontgen to conclude that X-rays impressed photographic plates and could therefore be revealed by this means. Even more important from a practical point of view was the discovery that X-rays can pass through numerous opaque bodies in the light. For example, X-rays pass through muscles, projecting only the shadow of bones onto the screen or photographic plate. By dosing the X-rays appropriately, it is possible to get X-rays not only of the skeleton but also of other internal organs, providing the doctor with a valuable means of investigation.

Rontgen's studies led to the first X-ray in history, the skeleton of a left hand with a ring finger wedding ring (the hand was that of his wife). Subsequent studies made it possible to ascertain that the X-rays are not deflected either by the action of electrical charges or by magnets, behaving in this respect in the strict manner of visible light. This fact and the discovery of other phenomena made it possible to conclude that X-rays are radiations of the same type as visible light (i.e. electromagnetic radiations), with a much shorter wavelength.

It also turned out that the smaller the wavelength of the X-rays, the more penetrating they are. With great energy, X-rays can cause chemical reactions and tear electrons from atoms and molecules, transforming them into ions. The discovery of X-rays, which was fundamental for later studies of radioactivity, won Rontgen the Nobel Prize in Physics in 1901.