At the interface between two materials having refractive indices ${n_1}$ and ${n_2}$ , the critical angle for reflection of an em wave is ${\theta _{1c}}$. 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 ${\theta _{2C}}$. If ${n_3} > {n_2} > {n_1};\frac{{{n_2}}}{{{n_3}}} = \frac{2}{5}$ and $\sin {\theta _{2C}} - \sin {\theta _{1C}} = \frac{1}{2}$, then ${\theta _{1C}}$ is
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