The Rydberg constant is a fundamental physical constant that characterizes the wavelengths of spectral lines in many chemical elements, particularly hydrogen. It is named after the Swedish physicist Johannes Rydberg, who formulated a formula in the 1880s to predict the wavelengths of the spectral lines of hydrogen.
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The inverse-length constant in the infinite-nuclear-mass hydrogenic spectrum, . A nucleus of finite mass replaces it by , where is the electron-nucleus reduced mass. Some spectroscopic fits call an effective Rydberg constant; the charge-inclusive and reduced-mass-only conventions must be distinguished.