A glass semicylinder with n = 1.5 is positioned such that its flat face is horizontal, as...

A glass semicylinder with n = 1.5 is positioned such that its flat face is horizontal, as shown in Fig. 1, and its horizontal surface supports a drop of oil, as also shown. When light is directed radially toward the oil, total internal reflection occurs if θ exceeds 60°. What is the index of refraction of the oil?

Figure 1

Oil drop on the flat surface of a glass semi-cylinder.