\( \Rightarrow \frac{{{\tau _1}}}{{{\tau _2}}} = \frac{{\sin {\theta _1}}}{{\sin {\theta _2}}} \)
\(\Rightarrow \frac{\tau }{{\tau /2}} = \frac{{\sin 90}}{{\sin {\theta _2}}}\)
\( \Rightarrow \sin {\theta _2} = \frac{1}{2} \Rightarrow {\theta _2} = {30^o}\)
\(==>\) angle of rotation \( = 90 - 30 = 60^\circ \)