a Swedish physicist particularly noted for his study of light, especially spectrum analysis. In 1853, Ångström demonstrated that the electric spark yields two superposed spectra, one from the metal of the electrode and the other from the gas through which it passes. His studies of the solar spectrum led to his theory that hydrogen is present in the Sun's atmosphere (1862). He was the first to examine the spectrum of the aurora borealis in 1867. Then, in 1868 he was able to map the solar spectrum. The angstrom, named for him, is a unit of length used to measure light waves (1 angstrom= 10-10 meters). He also performed important work concerning heat conduction; he devised a method of measuring thermal conductivity and showed it to be proportional to electrical conductivity.