Animation showing how electron transitions within a hydrogen atom produce the Balmer 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 lower levels, 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=3 and n=2 emits radiation with a wavelength of 656.3 nanometres, in the red part of the visible spectrum. It is this transition that produces the pink colour of emission nebulae in space. The larger drop from n=6 to n=2 has the shorter wavelength (and thus higher energy) of 410.2nm, in the violet part of the spectrum. The complete set of transitions from excited states to n=2 form the Balmer series. As the energy levels of the shells are constant, the wavelengths are always the same. See clip K004/4145 for the Lyman series, an analogous set of emission lines in the ultraviolet, due to transitions to n=1.