Question

The distance between a convex lens and a plane mirror is $40cm$. The parallel rays incident on the convex lens after reflection from the mirror form image at the optical centre of the lens. Focal length of the lens will be

• A. $80cm$
• B. $40cm$
• C. $30cm$
• D. $20cm$

Answer 1

The correct option is A $80cm$


First image due to parallel beam forms at $I_1$ i.e focal length of convex lens.

Considering $I_1$ as object for plane mirror, image has to form at $O$.

For Plane mirror,
distance of object from mirror $=$ distance of image from mirror
$\Rightarrow 40cm=(f-40)cm\Rightarrow f=80cm$