A parallel plate capacitor having plates of area S and plate separation d, has apacitance  $C_1$ in air. When two dielectrics of different relative permittivities $({\epsilon }_1=2and{\epsilon }_2=4)$ are introduced between the plates as shown in the figure, the capacitance becomes  $C_2$. The ratio $\frac{C_2}{C_1}$is