Define the term principal axis of a 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).

Explain the optical centre of a lens with the help of proper diagram(s).

State difference between a convex and a concave lens in their —

(a) appearance, and

(b) action on the incident light.