Question

Define principal axis, principal focus and focal length of a convex lens.

Answer 1

 An imaginary line that passes through the optical center and the center of curvature of both faces of the lens, and is perpendicular to the faces is known as the principal axis.
Focus is a point on the principal axis where all the rays that are parallel to the principal axis meet after refraction from the lens.
The focal length may be defined as the distance between the optical centre and principal focus of the lens.