What will the magnification be if an object is placed at the focus of a concave mirror?

Correct answer: d extremely magnified

At F the reflected rays become parallel and the image is at infinity. The nearer the object comes to F the larger the image; at F it has no limit, so the board's answer is extremely magnified.

Why the other options are wrong

• a equal: This is not the answer: equal size (m = 1) comes when the object is at the centre of curvature, not at the focus.
• b magnified: This is not the answer: a magnified image of limited size comes when the object is between C and F. At the focus the image is at infinity, so the magnification has no limit.
• c diminished: This is not the answer: a diminished image comes when the object is beyond C. Bringing the object towards the focus makes the image larger, not smaller.