What type of mirror is used to obtain a real 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 $\eta_{21}$?

The magnification of a mirror is $+1$. What does it mean?

The velocity of light in a vacuum is $....$

What can be said about the magnification of an image formed by a convex mirror?

What are the advantages and disadvantages of using a convex mirror for seeing traffic at the rear?