# A ray of light strikes a material slab at an angle of incident $60^{\circ}$ . If the reflected and refracted rays are perpendicular to each other, the refractive index of the material is
$\begin{array}{1 1} \large\frac{1}{\sqrt 3} \\ \frac{1}{\sqrt 2} \\ \sqrt {2} \\ \sqrt {3} \end{array}$