This Week in History: Wilhelm Roentgen, discoverer of X rays, was born March 27, 1845

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In 1895, the German physicist Wilhelm Roentgen discovered an invisible form of energy that he called X rays. Roentgen did not quite understand what these mysterious rays were—hence the mysterious-sounding “X” in their name—but scientists soon learned that they are a form of electromagnetic radiation. Such radiation includes the light we see with our eyes, as well as invisible radio waves. Roentgen’s discovery earned him a Nobel Prize in physics in 1901 and revolutionized the practice of medicine. Today, scientists use X rays to “see” the hidden forms of the universe, ranging from the structure of microscopic atoms to the energy streaming out from the centers of distant galaxies.

Roentgen was born on March 27, 1845, in Lennep (now Remscheid), Germany. He discovered X rays while working as a professor at the University of Wurzburg. Roentgen was experimenting with a Crookes tube, an early electric device that passes a current through an airless glass tube. Roentgen covered the tube with black paper. When he turned on the current, an image formed on a nearby photographic plate. He concluded that some kind of energy—X rays—must have come from the tube, passed straight through the black paper, and formed the photograph. X rays pass easily through flesh and clothing, but are blocked by bone and metal. Roentgen later used this property of X rays to photograph the bones in his wife’s hand.

Electromagnetic radiation takes the form of waves. Like water waves, electromagnetic waves have a wavelength—the distance between two wave “crests.” The longer the wavelength, the less energy the waves carry. Radio waves have the longest wavelength, followed by infrared waves, visible light, and ultraviolet waves. X rays have shorter wavelengths—and thus more energy—than ultraviolet waves. In fact, X rays have so much energy that they are dangerous. They can ionize atoms, stripping away their electrons. Ionization can cause a great deal of damage to structures inside the living cells. For this reason, X rays are used sparingly, and people exposed to X rays usually shield themselves with lead covering, which absorbs most of the rays’ energy.

Today, doctors use X rays in much the same way that Roentgen did—to take pictures of structures inside human bodies. Airport security professionals also use X rays to look inside luggage for dangerous objects. But X rays also shed light—albeit invisible—on structures far too small to see, such as the arrangement of atoms in crystals and the shape of molecules inside living cells. For example, the twisting-ladder shape of DNA—a complex molecule that encodes hereditary information—was first shown via X rays in 1951 by the British chemist Rosalind Franklin. On a much larger scale, astronomers study the X rays given off by stars and black holes give during violent cosmic events. The Chandra X ray observatory, a telescope launched into space in 1999, has provided some of the clearest pictures of the otherwise invisible X rays that flow throughout the universe.

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