[QUE/EM-06008] --- EM-PROBLEM --- Spherical cavity in a medium

A uniform electric field \(E_m\) is set up in a medium of dielectric constant \(\kappa\). Prove that the field inside a spherical cavity in the medium is given by \begin{equation*}   E = \frac{3\kappa E_m}{2\kappa+1} \end{equation*}