$(i)$ 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.

(N/A) $(i)$ Principal focus: $A$ number of rays parallel to the principal axis,after reflection from a concave mirror,meet at a point on the principal axis which is called the principal focus.
$(ii)$ $A$ convex mirror always forms an erect and diminished image. It provides a wider field of view because it is curved outwards,allowing the driver to see a much larger area behind the vehicle.
$(iii)$ To obtain an erect image with a convex lens,the object must be placed between the optical center $(O)$ and the principal focus $(F_1)$. The image formed is virtual,erect,and magnified,as shown in the diagram.