Define the term focal length of a lens.

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

A lens has two focal length.

1. First focal length — the distance from the optical centre O of the lens to its first focal point F₁ is called the first focal length.
2. Second focal length — the distance from the optical centre O of the lens to its second focal point F₂ is called the second focal length.