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 leng. 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

Using the lens formula `(1)/(f)=(1)/(v)-(1)/(u)`
Given, `v=d`, for equal size image `|v|=|u|=d`
By sign convention `u=-d`
`therefore" "(1)/(f)=(1)/(d)+(1)/(d) or f=(d)/(2)`