Animation showing how electron transitions within a hydrogen atom produce the Lyman series, a set of emission lines in the spectrum of hydrogen. A hydrogen atom is shown at left, opening to reveal its first six electron shells. The shell closest to the nucleus, n=1, is the lowest energy shell, called the ground state. The atom also has other shells, n=2, n=3 and so on, at successively higher energies. The electron in a hydrogen atom can often be found in an excited state, an energy level above n=1. When it falls from the higher level back to the ground state, it emits radiation at a characteristic frequency and wavelength, which depends on the energy difference between the shell and the ground state. As seen here, the relatively short transition between n=2 and n=1 emits radiation with a wavelength of 121.6 nanometres. The larger drop from n=3 to n=1 has the shorter wavelength (and thus higher energy) of 102.6nm. The complete set of transitions from excited states to the ground state form the Lyman series. As the energy levels of the shells are constant, the wavelengths are always the same, and these wavelengths are all in the ultraviolet part of the spectrum. See clip K004/4146 for the Balmer series, an analogous set of visible light emission lines due to transitions to n=2.