At the interface between two materials having refractive indices...

At the interface between two materials having refractive indices $n_1$ and $n_2$, the critical angle for reflection of an em wave is (\theta_{1 C}). The $n_2$ material is replaced by another material having refractive index $n_3$ such that the critical angle at the interface between $n_1$ and $n_3$ materials is (\theta_{2 $\mathrm{C}$}). If (\mathrm{n}}_{3}\gt \mathrm{n}}_{2}\gt\mathrm{n}}_{1} ; \frac{\mathrm{n}_2}{\mathrm{n}_3}=\frac{2}{5}) and (\sin \theta_{2 $\mathrm{C}$}-\sin \theta_{1 $\mathrm{C}$}=\frac{1}{2}), then (\theta_{1 $\mathrm{C}$}) is