A lamp is 5 m from a wall. Find the focal length of a concave mirror which will form a four times magnified image of the lamp on the wall.

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- 4m

20m

10m

answer is A.

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Detailed Solution

Concept: A mirror is a reflecting surface that reflects light and creates an actual or fictitious picture. When an object is placed in front of a mirror, the mirror reflects the image of the same thing. The incident rays are coming from the item, and the reflected rays are what create the image. The classification of the pictures as real or virtual depends on where the light beams cross. Real pictures happen when light rays truly cross, but virtual images happen when light rays from a point appear to diverge.
Object distance,

The image is magnified 4x, and if the image is a real image, it is known that the concave mirror always produces a magnified image, so

.
Here, the relationship between the magnification of the mirror and other quantities is given as follows.

, assign a value,
Question Image

Next, you need to find the focal length. Use a mirror formula:
Question Image

Hence, the correct answer option (1) .

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