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:
