A body is locatyed on a wall. Its image of equal size is to be obtained on 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

Options

A. Only `(d)/(4)`
B. Only `(d)/(2)`
C. More than `(d)/(4)` but less than `(d)/(2)`
D. Less than `(d)/(4)`

Answer

B

Explanation

The lens formula can be written as
` (1)/(f)=(1)/(v)+(1)/(u)` ....(i)
Given `v=d`
For equal size image
`|v|=|u|=d`
By sign convention `u = -d`
`:. (1)/(f)=(1)/(d)+(1)/(d)` or `f=(d)/(2)`.