When a concave mirror produces a virtual and magnified image of an object,what is the object distance?

An incident ray falls normally on a glass slab. What is the angle of refraction (in $°$)?

What is a prism?

Two plane mirrors are at right angles to each other. $A$ man stands between them and combs his hair with his right hand. In how many images will he be seen using his left hand?

As the velocity of light increases,the refractive index of the medium decreases. Light enters from air to water having a refractive index of $4/3$. Find the speed of light in water. The speed of light in vacuum is $3 \times 10^{8} \text{ m s}^{-1}$.

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 $\eta_{21}$?