What should be the value of refractive index n of a glass rod placed in air, so that the light entering through the flat surface of rod does not cross the curved surface of rod.

Question

$(a)\; n > \frac{1}{\sqrt 2} \\ (b)\;n < \frac{1}{\sqrt 2} \\ (c)\; n > \sqrt 2 \\ (d)\; n < \sqrt 2 $