Evaluate the following expressions. Your answer must be an angle in radians and in the interval $[-\frac {\pi}{2} , \frac {\pi}{2} ]$.

(a) $\sin^{-1}(-\frac{\sqrt{3}}{2}) =$

(b) $\sin^{-1}(\frac{1}{2}) =$

(c) $\sin^{-1}(\frac{\sqrt{3}}{2}) =$