Light enters from air to kerosene having refractive index of $1.47$. What is the speed of light in kerosene, if the speed of light in air is $3\times {10}^8 \mathrm{m} {\mathrm{s}}^{-1}$?

Write the sign convention for reflection by spherical mirrors.

Why convex mirrors are used as rear-view mirrors in vehicles?

If the object is placed at a principal focus $\mathrm{F}_{1}$ of a convex lens, draw the ray diagram for the image formation.

The diagram shows an object '$O$', and its image '$I$' formed by a lens. In the diagram, draw the lens and the rays to show how the image is formed. Mark focus $F$, of the lens. Name the lens.

Mention any two achievements of Hubble Telescope.

Define Power of lens. Give its unit.