Define the term principal foci of a convex lens and illustrate your answer with the aid of proper diagrams.

Define the term principal foci of a concave lens and show them with the help of proper diagrams.

Draw a diagram to represent the second focus of a convex lens.

A ray of light incident at a point on the principal axis of a convex lens passes undeviated through the lens.

(a) What special name is given to this point on the principal axis?

(b) Draw a labelled diagram to support the answer in part (a).