An Etymological Dictionary of Astronomy and Astrophysics
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One of the postulates used in the → Bohr model, whereby the atom emits (absorbs) a quantum of electromagnetic energy (→ photon) when the electron passes from an orbit with a greater (lesser) n value to one with a lesser (greater) value. The energy of the quantum is equal to the difference between the energies of the electron on its orbits before and after the transition or "jump": hν = ε₁ - ε₂, where h is the → Planck's constant and ν the frequency of the transition.

→ Bohr; → third; → postulate.