A ray of light is incident on glass sphere of refractive index $\large\frac{3}{2}$. The ray incident at two surfaces after travelling through the sphere emerges tangential to the surface of the sphere. What must be the incident angle.

Question

$(a)\;\tan ^{-1} 2/3 \\ (b)\;\sin ^{-1} 2/3 \\ (c)\;90^{\circ} \\ (d)\;\cos^{-1} 1/3$