Question

In an experiment a convex lens of focal length $15cm$ is placed coaxially on an optical bench in front of a convex mirror at a distance of $5cm$ from it. It is found that an object and its image coincide, if the object is placed at a distance of $20cm$ from the lens. The focal length of the convex mirror is

• A. 27.5 cm
• B. 20.0 cm
• C. 25.0 cm
• D. 30.5 cm

Answer 1

Answer: (A)

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
and $v=60cm$
$2f=55$ and $f=27.5cm$