(symbol R_∞), fundamental constant of atomic physics that appears in the formulas developed (1890) by the Swedish physicist J. R. Rydberg (Rydberg, Johannes Robert), describing the wavelengths or frequencies of light in various series of related spectral lines, such as those emitted by hydrogen atoms. The value of this constant is based on the premise that the nucleus of the atom emitting light is exceedingly massive compared with a single orbiting electron (hence the infinity symbol ∞).

The value of the Rydberg constant R_∞ is 1.097373177 × 10⁷ per metre, or 109737.31 per centimetre. When used in this form in the mathematical description of series of spectral lines, the result is the number of waves per unit length, or the wave numbers. Multiplication by the speed of light yields the frequencies of the spectral lines.