= Transition frequency
{title2=$\nu$}
The <transition frequency> between two stationary <energy levels> is the <positive frequency> satisfying $h\nu=|E_f-E_i|$. In <absorption>, the system gains this <energy> from the <radiation>; in <emission>, it loses it. The corresponding <angular frequency> is $\omega=2\pi\nu$.
An <energy> difference specifies where a <spectral line> can occur, but does not by itself guarantee that the transition is strong or allowed. The interaction and its <selection rules> determine the transition <amplitude>. External fields, motion and finite lifetimes can shift or broaden the measured line.
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