What is the unit of refractive index?

Magnification is the ratio of which of the following?

$(a)$ Write the relation between $u, v, f$ for lenses and for mirrors,where $u, v, f$ are object distance,image distance,and focal length respectively.
$(b)$ The magnification produced by a concave mirror is $m = +4$. Write the information about the image given by this statement.
$(c)$ Draw a ray diagram for the following and show the formation of the images in the case of a concave mirror when the object is placed:
$(i)$ Between the pole and the focus point.
$(ii)$ At the centre of curvature.

What is the centre of a curved mirror called?

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 radius of curvature of a spherical mirror is half of the focal length.