9. For Lens, the position of objects and it's image are interchangeable i.e.if the object is placed at a position previously occupied by its image, then the new image will be formed at the position previously occupied by the object. The pair of object and image position are known as Conjugate pair and the planes passing through these points, perpendicular to the axis are known as conjugate planes.
10. In any optical system, the ratio of the transverse dimension of the final image to the corresponding dimension of the object is called the lateral magnification.
11. The power (P) of the lens is defined as the inverse of its focal length (f ). If the focal length is given in meter, then the unit of power is diopter.

                                        

Sign Convention:

1. All figures are drawn with the light traveling from left to right.
2. All object distances (u) are positive when measured left to the vertex and negative when measured right to the vertex.
3. All Image distances (v) are positive when measured right to the vertex and negative when measured left to the vertex.
4. Focal length is positive for a converging lens and negative for a diverging lens.
5. Object and Image dimensions are positive when measured upward from the axis and negative when measured downwards