The constant is expressed for either hydrogen as formula_2, or at the limit of infinite nuclear mass as formula_1. In either case, the constant is used to express the limiting value of the highest wavenumber (inverse wavelength) of any photon that can be emitted from an atom, or, alternatively, the wavenumber of the lowest-energy photon capable of ionizing an atom from its ground state. The hydrogen spectral series can be expressed simply in terms of the Rydberg constant for hydrogen formula_2 and the Rydberg formula.