(Ry), a subsidiary unit of energy used in atomic physics and optics. It was named in honor of J. R. Rydberg. It is defined by the equation 1 Ry = hcR_∞, where h is Planck’s constant, c is the velocity of light, and R_∞ is the Rydberg constant; numerically, 1 Ry = 13.60 electron volts—that is, 1 Rydberg is equal to the ionization energy of an atom of hydrogen. The Rydberg is a convenient unit for measuring the energy of atomic states. In the Hartree atomic system of units, 1 Ry is equal to one-half of an energy unit; in the centimeter-gram-second system, 1 Ry = 2.1796 × 10^-11 erg.