Unpolarised light is incident on the boundary between two dielectric media, whose dielectric constants are 2.8 (medium -1) and 6.8 (medium -2),…

Unpolarised light is incident on the boundary between two dielectric media, whose dielectric constants are $\displaystyle 2.8$ (medium -$\displaystyle 1$) and $\displaystyle 6.8$ (medium -$\displaystyle 2$), respectively. To satisfy the condition, so that the reflected and refracted rays are perpendicular to each other, the angle of incidence should be $\displaystyle \tan ^{-1}\left(1+\frac{10}{\theta}\right)^{\frac{1}{2}}$ the value of $\displaystyle \theta$ is $\displaystyle \_\_\_\_$. (Given for dielectric media, $\displaystyle \mu_{\mathrm{r}}=1$ )