A body is located on a wall. Its image of equal size is

A body is located on a wall. Its image of equal size is to be obtained on a parallel wall with the help of a convex lens. The lens is placed at a distance 'd' ahead of second wall, then the required focal length will be

Using the lens formula ,	1	=	1	-	1
	f		v		u

Given v = d, for equal size image | v | = | u | = d
By sign convention u = –d

∴	1	=	1	+	1	or f =	d
	f		d		d		2

only	d
	4

only	d
	2

more than	d	but less than	d
	4		2

less than	d
	4