An observer can see through a pin-hole the top end of a thin rod of height h, placed as shown in the figure. The beaker height is 3h and its radius h. When the beaker is filled with a liquid up to a height 2h, he can see the lower end of the rod. Then the refractive index of the liquid is

$\sqrt{\frac{5}{2}}$

$\sin i /\sin r = 1/μ; r = 45°;$

$\sin i = 1/\sqrt{5} ⇒ μ = (5/2)^{1/2}$.