Axial magnification = – dv / du =( v / u) 2 =( f / f + u) 2 =( f-v / f) 2
Focal Length of a Convex Lens by Displacement Method
Focal length of the convex lens f =( a 2 – d 2) / 4a where, a = distance between the image pin and object pin and d = distance between two positions of lens.
The distance between the two pins should be greater than four times the focal length of the convex lens, i. e., a > 4f.
Height of the object O = √I 1 I 2
Cutting of a Lens
( i) If a symmetrical convex lens of focal length f is cut into two parts along its optic axis, then focal length of each part( a plano convex lens) is 2f. However, if the two parts are joined as shown in figure, the focal length of combination is again f.
( ii) If a symmetrical convex lens of focal length f is cut into two parts along the principal axis, then focal length of each part remains unchanged as f. If these two parts are joined with curved ends an one side, focal length of the combination is f / 2. But on joining two 2 parts in opposite sense the net focal length becomes