Question

A concave mirror has a focal length of 20 cm. There are three points P, Q, and R at a distance of 10 cm, 20 cm and 30 cm from the pole of the mirror. At what point should the object be kept so that the size of its image is larger than the size of the object itself.

• A. P & Q
• B. Q & R
• C. R & P
• D. P, Q & R

Answer 1

The correct option is D P, Q & R

Let's recall the different cases of image formation. We know that, when the object is at infinity, the image is just a point at the focus of the mirror. As we bring our object closer to the mirror, the size of the image starts increasing, when the object is at the center of curvature, the size of the image is same as the size of the object.
When we bring the object further closer to the mirror, the image size will be bigger than the object size after that. Also in the case of the object being between the pole and the focus of the mirror, it forms a virtual image, which is larger than the object.
So we can say that whenever the object is between the center of curvature of the mirror and the pole, the size of the image will be larger than the size of the object. So, if the focal length is 20 cm, the center of curvature will be at 40 cm. Hence if we keep the object anywhere up to a distance of 40cm from the mirror, we will get the image size larger than the object. Hence at all the three points P, Q, and R, its possible.