A sequence of absorption or emission lines in the ultraviolet part of the spectrum, due to hydrogen. They range from Lyman-α‎ at 121.6 nm towards shorter wavelengths, the spacing between the lines diminishing as they converge on the Lyman limit at 91.2 nm. The Lyman series is caused by electron jumps between the ground state and higher levels of the hydrogen atom. The term is also used to describe certain lines in the spectrum of singly ionized helium. The He II Lyman lines have almost exactly one-quarter the wavelength of their hydrogen equivalents: for example, He II Lyman-α‎ is at 30.4 nm, and the corresponding Lyman limit is at 22.7 nm. It is named after the American physicist Theodore Lyman (1874–1954). See also hydrogen spectrum.