An infinitely long uniform line charge distribution of charge per unit length $\lambda$ lies parallel to the $y$-axis in the $y-z$ plane at $z=\frac{\sqrt{3}}{2} a$ (see figure). If the magnitude of the flux of the electric field through the rectangular surface $A B C D$ lying in the $x-y$ plane with its centre at the origin is $\frac{\lambda L}{n \varepsilon_0}\left(\varepsilon_0=\right.$ permittivity of free space), then the value of $n$ is :