A ray of light traveling in air is incident at angle $\theta_{a}$ on one face of a $90.0^{\circ}$ prism made of glass. Part of the light refracts into the prism and strikes the opposite face at point $A$ (Fig. $\mathbf{P 3 3 . 4 2}$ ). If the ray at $A$ is at the critical angle, what is the value of $\theta_{a} ?$