The figure shows a parallel-plate capacitor having square plates of edge length ‘$a$’ and plate-separation $d$. The gap between the plates is filled with a dielectric of dielectric constant $K$ which varies as  $K=K_0+\alpha x$ where $K_0$ and $\alpha$ are constants and $x$ is the distance from the left end. Capacitance of this capacitor is $\frac{{\epsilon }_0{a}^2}{d}\left[K_0+\frac{\alpha a}{n}\right]$. The value of ‘$n$’ is