An object is placed at the focus of a concave lens. Where will be image ?

Options

A. `(f)/(2)`
B. `-(f)/(2)`
C. 2f
D. `-(f)/(4)`

Answer

B

Explanation

u = -f and for a concave lens, f is -ve.
`because (1)/(v)-(1)/(u)=(1)/(f)`
`therefore (1)/(v)=(1)/(f)+(1)/(u)=-(1)/(f)-(1)/(f)= -(2)/(f)`
`therefore v= -(f)/(2)`