Rotating-wave approximation

The rotating wave approximation is an approximation used in atom optic and magnetic resonance. In this approximation, terms in a Hamiltonian which oscillate rapidly are neglected. This is a valid approximation when the applied electromagnetic radiation is near resonance with an atomic resonance, and the intensity is low. Explicity, terms in the Hamiltonians which oscillate with frequencies $\omega _{l}+\omega _{0}$, are neglected while terms which oscillate with frequencies $\omega _{l}-\omega _{0}$ are kept, where $\omega _{l}$ is the light frequences, and $\omega _{0}$ is a transition frequency.