Question

Two big mirrors A and B are fitted side by side on a wall. A man standing at such a distance from the wall that he can see the erect image of his face in both the mirrors. When the man starts walking towards the mirrors, he finds that the size of his face in mirror A goes on increasing but that in mirror B remains the same. Choose the right option.
A) Mirror A is concave and mirror B is convex.
B) Mirror A is plain and mirror B is concave.
C) Mirror A is concave and mirror B is plane.
D) Mirror A is convex and mirror B is plane.

Answer 1

Hint: We need to understand the relation between the object distance, the type of image formed and the image distance to analyse and conclude the type of mirrors used on the wall where the man sees his reflection, which gives us the solution.

Complete answer:
We should know the basic types of mirrors and the type of image formed by these mirrors with respect to the object distance. We can discuss each of the mirrors for ease.
1.Plane mirrors: These mirrors give a virtual and erect image of the size as that of the object when kept anywhere in its vicinity. The image formed will suffer from lateral inversion, otherwise the image and the object look the same.
2.Concave mirrors: These mirrors give real and inverted images for all the points from focal point of the mirror to infinity. The mirror gives a virtual and erect image when the object is between the focal point and the mirror. During this time, the image is magnified also.
3.Convex mirrors: These mirrors form images which are always virtual and erect. The image formed is always smaller than the object. The image size decreases as the object approaches the mirror.
From the above discussions, we can understand the types of mirrors used as A and B.
A has an erect image and the size of image increases as the object approaches the mirror. Therefore, the mirror is a concave mirror.
The below diagram shows the virtual, erect and enlarged image formed by the mirror A.
                

B has an erect image and the image size is constant as the object approaches the mirror. Therefore, the mirror is a plane mirror.
           

The correct answer is option C.

Note:
We need to be careful about the erect nature of the image as given in the problem. All the mirrors can give virtual and erect images at some point. The size of the image is the factor that distinguishes the three mirrors from each other in this case.