A material having an index of refraction $n$ is surrounded by vacuum and is in the shape of a quarter circle of radius $R(\text { Fig. } \mathrm{P} 35.63) .$ A light ray parallel to the base of the material is incident from the left at a distance $L$ above the base and emerges from the material at the angle $\theta .$ Determine an expression for $\theta$ in terms of $n, R,$ and $L .$