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. Ray 2 on the other hand is along the principal axis and is normal on the rectangular glass block. This ray passes un-deviated along the principal axis.