An object is there on a wall. Now by help of convex lens, an image of object is obtained of same size as of object on the opposite parallel wall. If the lens is at distance d from second wall, then what should be the focal length of lens ?

Options

A. `d/4`
B. `d/2`
C. more than `d/4` but less than `d/2`
D. less than `d/4`

Answer

B, D

Explanation

By substituting v=d in lens equation =`1/f=1/v-1/u` and for image of same size of object,
Taking |v|=|u|=d
u=-d (From sign convention)
`therefore1/f=1/d=((1)/(-d))`
=`1/d+1/d`
`therefore1/f=2(1/d) thereforef=d/2`