Light is incident along the normal to face $A B$ of a prism that has a refractive index $n$ and an angle $\alpha=30^{\circ}$, as shown in Figure $23.54 .$ When the prism is in air, the light undergoes total internal reflection. However, when the prism is immersed in water, a faint refracted ray emerges at an angle of $80^{\circ}$ to the normal of face $A C$. (a) Determine $n$. (b) What material was used to make the prism?