Karl Siegbahn was a Swedish physicist who was awarded the 1924 Nobel Prize in Physics for research into X-ray spectroscopy. Siegbahn improved techniques for measuring the wavelength of X-rays and used these new methods to discover groups of X-rays both less penetrating and with longer wavelengths than had previously been thought possible. This discovery supported the shell theory of atomic structure of the Danish physicist Niels Bohr (see Bohr atom). In 1924 he also proved the similarity between X-rays and visible light by showing that X-rays could be refracted by a prism.