The second focus F 2 of a concave lens that point on the principle axis where the parallel light rays impinging on the lens appear to converge on the same side of the lens. This point is called the second focus. The distance between the optical centre O and the first focal point is known as the first focal length f 1 . The distance between the optical centre O and the second focal point is known as the second focal length f 2 . Similarly, a vertical plane passing through F 1 is known as the first focal plane and a vertical plane passing F 2 through is called the second focal plane.
Figure below shows how light rays are focused by a convex lens.
A convex lens may be considered to be made up of two prisms and a solid glass block in the centre. Prisms divert or refract light rays toward the direction of their bases. The prisms have their bases towards the optical centre. Ray 1 is parallel to the principal axis. F. Similarly for ray 3.