Question

Define focal length of a lens.

Answer 1

The distance of the principal focus from the optical centre of a lens is called its focal length.

The point on the principal axis, at which the rays parallel to the principal axis, after refraction from the lens, converge at or appear to diverge from is called the principal focus of the lens.

For the convex lens in (a),

$F_2$ is the principal focus and $O{F}_2$ is the focal length.

For the concave lens in (b),

$F_1$ is the principal focus and $O{F}_1$ is the focal length.