خطا
$\left. \begin{matrix} \operatorname{Sin}{{60}^{{}^\circ }}\times {{n}_{1}}=\operatorname{Sin}{{\theta }_ {r}}\times {{n}_{2}} \\ \operatorname{Sin}{{\theta }_{r}}\times {{n}_{2}}=\operatorname{Sin}{{{{\theta }'}}_ {r}}\times {{n}_{3}} \\ \end{matrix} \right\}\Rightarrow \operatorname{Sin}{{60}^{{}^\circ }}\times {{n}_{1}}= \operatorname{Sin}{{{\theta }'}_{r}}\times {{n}_{3}}$ $\Rightarrow \frac{\sqrt{3}}{2}\times 1=\operatorname{Sin}{{{\theta }'}_{r}}\times \sqrt{3}\Rightarrow \operatorname{Sin}{{{\theta }'}_{r}}=\frac{1}{2}\Rightarrow {{{\theta }'}_{r}}={{30}^{{}^\circ }}$ $\Rightarrow \theta ={{90}^{{}^\circ }}-{{30}^{{}^\circ }}={{60}^{{}^\circ }}$