Rydberg constant
The constant is expressed for either hydrogen as , or at the limit of infinite nuclear mass as . 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  and the Rydberg formula.