$A$ convex mirror always forms a virtual and erect image.

The refractive indices of medium $(1)$ and medium $(2)$ are $\eta_{1}$ and $\eta_{2}$ respectively,and the velocities of light in these media are $v_{1}$ and $v_{2}$ respectively. What is the formula to find the refractive index of medium $(2)$ with respect to medium $(1)$,denoted as $\eta_{21}$?

Where is the image formed when an object is placed at a distance of $2f$ in front of a convex mirror?

The compound microscope consists of two convex lenses of $5 \, cm$ and $20 \, cm$ focal length. Which of them will be the objective and the eyepiece?

What is the absolute refractive index of water?

Which of the following conditions must be satisfied to obtain a magnified image of an object using a concave mirror?