The image formed by a lens is always erect and diminished. What is the nature of the lens?

The refractive index of water with respect to air is $\frac{4}{3}$ and the refractive index of glass with respect to air is $\frac{3}{2}$. What is the refractive index of glass with respect to water?

$(a)$ Explain the following terms related to spherical lenses:
$(i)$ Optical centre
$(ii)$ Centres of curvature
$(iii)$ Principal axis
$(iv)$ Aperture
$(v)$ Principal focus
$(vi)$ Focal length
$(b)$ $A$ converging lens has a focal length of $12 \, cm$. Calculate at what distance should the object be placed from the lens so that it forms an image at $48 \, cm$ on the other side of the lens.

What is the distance between the pole and the principal focus of a spherical mirror called?

What is the unit of refractive index?

When light of two colours $A$ and $B$ is passed through a plane boundary; $A$ is bent more than $B$. Which colour travels more slowly in the second medium?