Q.A concave mirror of curvature 40 cm, used for shaving purpose produces image of double size as that of the object. Object distance must be (A) 10 cm only (B) 20 cm only (C) 30 cm only (D) 10 cm or 30 cm

For a spherical mirror (concave or convex) with radius of curvature $R$, the focal length is $f=R/2$ -- by the Cartesian convention, positive for a convex mirror (virtual focus, behind the mirror) and negative for a concave mirror (real focus, in front). For a small-aperture mirror, object distance $u$, image distance $v$ and focal length $f$ satisfy the MIRROR FORMULA $\frac1f=\frac1v+\frac1u$. The …