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 \( \tan^{-1}\!\left(\left(1+\frac{10}{\theta}\right)^{\frac{1}{2}}\right) \) the value of \( \theta \) is _______. (Given for dielectric media, \( \mu_r=1 \))