X rays are electromagnetic radiation with short wavelengths (10^–3 nanometers) and a great amount of energy. They were discovered in 1898 by William Conrad Roentgen (1845–1923). X rays are frequently used in medicine because they are able to pass through opaque, dense structures such as bone and form an image on a photographic plate. They are especially helpful in assessing damage to bones, identifying certain tumors, and examining the chest—heart and lungs—and abdomen. A major disadvantage of X rays as a diagnostic tool is that they provide little information about the soft tissues. Since they only show a flat, two-dimensional picture, they cannot distinguish between the various layers of an organ, some of which may be healthy while others may be diseased.