In a viscously damped vibratory system with harmonic excitation, if $\omega _{1}$ and $\omega _{2}$ are the half power points, $\omega _{n}$ is the natural frequency and $\xi$ is the damping factor which one of the following is correct?
(a) $\left ( \omega _{2}-\omega _{1} \right )/\omega _{n}=2\xi$
(b) $\left ( \omega _{2}+\omega _{1} \right )/\omega _{n}=2\xi$
(c) $\left ( \omega _{2}-\omega _{1} \right )/\omega _{n}=\xi ^{2}$
(d) $\left ( \omega _{2}-\omega _{1} \right )/3\omega _{n}=\xi$