\(\cfrac { 1 }{ v } -\cfrac { 1 }{ u } =\cfrac { 1 }{ f } \) or \(\cfrac { 1 }{ v } =\cfrac { 1 }{ f } +\cfrac { 1 }{ u } \) 
convex lens : f > 0 ; u < 0 (object on the left)
For 0 < I u I \(\cfrac { 1 }{ v } =\cfrac { 1 }{ f } -\cfrac { 1 }{ |u| } <0\) i.e. u < 0 (image on left; virtual)
Also for this case, \(\cfrac { 1 }{ \left| v \right| } <\cfrac { 1 }{ |u| } .i.e.\left| u \right| <\left| v \right| \) (image enlarged)

(i) A lens is formed by a transparent material bounded between two spherical surfaces or one plane and another spherical surface.
(ii) In a thin lens, the distance between the surfaces is very small. If there are two spherical surfaces, then there will be two centers of curvature C₁ and C₂ and correspondingly two radii of curvature R₁ and R₂.
(iii) A plane surface has its center of curvature C at infinity and its radius of curvature R is infinity (R = ∞).

(i) When light rays incident on a spherical mirror, after reflection, the rays converge at a point on the principal axis for concave mirror or appear to diverse from a point for convex Mirror.
(ii) This point is called the focus or focal point (F) of the mirror.

(i) The line joining the pole and the centre of curyature is called the principal axis of the mirror.
(ii) The light ray travelling along the principal axis towards the mirror after reflection travels back along the same principal axis. It is also called optical axis.