Rydberg constant
In spectroscopy, the Rydberg constant is a physical constant relating to the electromagnetic spectra of an atom. Its symbol is  for heavy atoms or  for hydrogen. The constant is named after the Swedish physicist Johannes Rydberg.  The constant first arose as an empirical fitting parameter in the Rydberg formula for the hydrogen spectral series.  Niels Bohr later showed that its value could be calculated from more fundamental constants via his Bohr model. ,  and electron spin g-factor are the most accurately measured physical constants.