A transparent cube of $15 \mathrm{cm}$ edge contains a small air bubble. Its apparent depth, when viewed through one face, is $6 \mathrm{cm}$ and when viewed through the opposite face is $4 \mathrm{cm}$. Then, the refractive index of the material of the cube is

A plane mirror is placed at the bottom of the tank containing a liquid of refractive index, $\mu$. $P$ is a small object at a height, $h$ above the mirror. An observer, $O$ vertically above $P$ outside the liquid, sees $P$ and its image in the mirror. The apparent distance between these two will be