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