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. 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.