When a dielectric medium (like kerosene oil) is introduced between the charged plates, the material undergoes polarisation. This polarisation creates an induced electric field that acts in the opposite direction to the original external field produced by the charges on the plates . Consequently, the net electric field $\mathbf{E}$ inside the dielectric is reduced compared to the field in vacuum $\mathbf{E}_0$. The relationship is given by $E = \frac{E_0}{K}$, where $K$ is the dielectric constant of the medium . Since the dielectric constant for kerosene is greater than 1 ($K > 1$), the electric field decreases .