Question

A point object is placed at a distance of 25 cm on the axis of a convex lens of focal length 20 cm .On the other side of the lens, a convex mirror is placed at a distance of 50 cm from the lens such that the image formed by the combination coincides with the object itself . The focal length of the convex mirror is

• A. 10 cm
• B. 15 cm
• C. 20 cm
• D. 25 cm

Answer 1

The correct option is D 25 cm

$\frac{1}{v}-\frac{1}{u}=\frac{1}{20}$

$\frac{1}{v}-\frac{1}{(-25)}=\frac{1}{20}$

$v=100cm$ (@ centre of curvature)

$R=50$

$f=\frac{50}{2}=25cm$