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 by refracted rays are perpendicular to each other,the angle of incidence should be `tan^(-1)(1+(10)/(theta))^((1)/(2))` the value of `theta` is_______ (Given for dielectric media, `mu_(r)=1`