Question

An object is placed at the focus of a convex mirror. If its focal length is $20\, cm,$ the distance of mirror is

• A. 20 cm
• B. 10 cm
• C. 40 cm
• D. None of these

Answer 1

Answer: (B)

10 cm

Answer 2

$f$ is +ve for convex mirror.
Therefore, for convex mirror
$\frac{1}{v}-\frac{1}{20} =+\frac{1}{20}$
$\frac{1}{v} =\frac{1}{10}$
or $v =10\, cm$