The intensity of polarized light, $I$, passing through a rotating filter is modeled by the equation $I=I_0{\cos }^2\theta$ where $I_0$ is the maximum intensity and $\theta$ is the angle of the filter in radians. In a specific experiment, the maximum intensity is measured as $I_0=12$ units. Determine all values of $\theta$ in the interval $0\le \theta \le 2\pi$ for which the intensity $I$ is exactly $9$ units. Give your answers in terms of $\pi$.