An isolated parallel-plate capacitor carries a uniform surface charge density of magnitude \(\sigma\) on each plate. A dielectric slab with dielectric constant \(\kappa\) is placed between and parallel to the plates, partially filling the space so that a dielectric layer and an adjacent vacuum layer coexist between the plates. What is the ratio of the electrostatic energy density within the dielectric layer to that within the vacuum layer, \(u_{\text{diel}} / u_{\text{vac}}\)?