An infinitely long rod lies along the axis of a concave mirror of focal length f. The near end of the rod is distance `u gt f` from the mirror. Its image will have length

Options

A. `(f^2)/(u + f)`
B. `uf//(u - f)`
C. `(f^2)/(u - f)`
D. `uf//(u + f)`

Answer

C

Explanation

For concave mirror
`u = -u, f = -f`
using mirror formula
`v = (uf)/(u - f)`
Length of image `L = |v| - |f|`
`= (uf)/(u - f) = f`
Solving we get `L = (f^2)/( u - f)`