Let the field in the dielectric be $\vec{E}$ making an angle $\alpha$ with $\vec{n} .$ Then we have the boundary conditions, $E_{0} \cos \alpha_{0}=\varepsilon E \cos \alpha$ and $E_{0} \sin c_{0}=E \sin \alpha$
So $E=E_{0} \sqrt{\sin ^{2} \alpha_{0}+\frac{1}{\varepsilon^{2}} \cos ^{2} \alpha_{0}}$ and $\tan \alpha=\varepsilon \tan \alpha_{0}$
In the dielectric the normal component of the induction vector is $D_{n}=\varepsilon_{0} \varepsilon E_{n}=\varepsilon_{0} \varepsilon E \cos \alpha=\varepsilon_{0} E_{0} \cos \alpha_{0}$
$\sigma^{\prime}=P_{n}=D_{n}-\varepsilon_{0} E_{n}=\left(1-\frac{1}{\varepsilon}\right) \varepsilon_{0} E_{0} \cos \alpha_{0}$
or, $\quad \sigma^{\prime}=\frac{\varepsilon-1}{\varepsilon} \varepsilon_{0} E_{0} \cos \alpha_{0}$