A concave mirror has a focal length of 20 cm. A paraxial beam of light coming from a distant object forms its image at the focal point. If a small plane mirror is placed at the center of curvature with its reflecting side facing the focal point, where is the final image?

Question

Quizplus · Accepted Answer

When the small plane mirror is placed at the center of curvature, the light rays from the object that were originally going to converge at the focal point will now be reflected back along their original paths, parallel to the axis of the concave mirror.

A parallel beam of light incident on a concave mirror reflects back through the focal point of the mirror. So, the parallel beam of light starting from the mirror will pass through the focal point of the concave mirror.

Thus, the light rays will converge at a point that is twice the distance between the focal point and the center of curvature, which is 40 cm (20 x 2).

Therefore, the final image will appear 60 cm (40 + 20) in front of the mirror.

Hence, the answer is D.

A Concave Mirror Has a Focal Length of 20 Cm