A, B, and C are parallel-sided transparent media with refractive indices $n_1$, $n_2$, and $n_3$, respectively. They are arranged as shown in the figure. A ray is incident at an angle $\theta$ on the surface of separation of A and B, as shown in the figure. After refraction into medium B, the ray grazes the surface of separation of media B and C. Then, $\sin \theta = \ldots$