X rays are one of the most useful forms of energy. Their main uses have been in the field of medicine. X rays find wide use in medicine because they can pass through flesh and produce photographic images of what lies beneath the skin. X rays also have many uses in science. For example, researchers who investigate the structure of solid materials use X rays to determine the arrangement of atoms in crystals. Astronomers study X rays generated by stars and other heavenly objects to learn about the objects' structure and temperature.
The German physicist Wilhelm C. Roentgen discovered X rays in 1895. He called them X rays because at first he did not understand what they were. X is a scientific symbol for the unknown.
Scientists now know that X rays are one of several forms of electromagnetic radiation. Another form of such radiation is visible light. The various forms can be distinguished by their wavelength, the distance between successive crests of their waves. From the shortest wavelength to the longest, electromagnetic radiation consists of gamma rays, X rays, ultraviolet rays, visible light, infrared rays, and radio waves. Wavelength is related to energy: the shorter the wavelength, the higher the energy. Thus, X rays have a shorter wavelength, but more energy, than do waves of visible light.
X rays can produce biological, chemical, and physical changes in the substances they enter. For this reason, X rays can be dangerous. In human beings, an overdose of X rays may cause cancer, skin burns, anemia, or other serious conditions. But physicians also use X rays to treat cancer. X rays kill cancer cells more readily than they kill normal cells.
In X-ray machines used in medicine and science, a special tube that resembles a television picture tube produces X rays. Scientists also use intense, narrow beams of X rays generated by devices called synchrotrons. In nature, extremely hot stars and other heavenly objects emit (send out) X rays.
