Which of the following is the lens formula?

What is meant by the power of a lens? Give its $SI$ unit. When two or more lenses are placed in contact,what will be their combined power?

$(i)$ Define the principal focus of a concave mirror.
$(ii)$ Why do we prefer a convex mirror as a rear-view mirror in vehicles?
$(iii)$ We wish to obtain an erect image of an object using a convex lens. Draw a ray diagram to show the image formation in this case.

An object is placed at a distance of $25 \, cm$ away from a converging mirror of focal length $20 \, cm$. Discuss the effect on the nature and position of the image if the position of the object changes from $25 \, cm$ to $15 \, cm$. Justify your answer without using the mirror formula.

The path of a ray of light coming from air passing through a rectangular glass slab traced by four students are shown as $A, B, C$ and $D$ in the figure. Which one of them is correct?

With respect to air,the refractive indices of ice and rock salt are $1.31$ and $1.54$ respectively. Calculate the refractive index of rock salt with respect to ice.