If the angle of incidence on a plane mirror is $30^{\circ}$,what is the angle of reflection (in $^{\circ}$)?

For a concave mirror,if the object distance $u = \infty$,then the image distance $v = ..........$.

Two lenses have powers of $(a) +2 \text{ D}$ and $(b) -4 \text{ D}$. Find the nature and focal length of each lens.

The frequency,wavelength,or speed of light does not change on reflection.

If the absolute refractive indices of water, benzene, and sapphire are $1.33$, $1.50$, and $1.77$ respectively, then which medium has the maximum relative refractive index?

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 $\eta_{21}$?