Three immiscible transparent liquids with refractive indices $\frac{3}{2}, \frac{4}{3}$ and $\frac{6}{5}$ are arranged one on top of another. The depths of the liquids are $3 \mathrm{cm}, 4 \mathrm{cm}$ and $6 \mathrm{cm}$ respectively. The apparent depth of the vessel is

Two similar plano-convex lenses are combined in three different ways as shown in the adjoining figure. The ratio of the focal lengths in the three cases will be

In the figure, an air lens of radii of curvature $10 \mathrm{cm}$ $\left(R_1=R_2=10 \mathrm{cm}\right)$ is cut in a cylinder of glass $\left(\mu =\frac{3}{2}\right)$. The focal length and the nature of the lens is