Question

___ lens is used in wide-angle spyhole is doors.
(A) Concave mirror
(B) Convex mirror
(C) Concave lens
(D) Convex lens

Answer 1

To answer this question, we need to look into the properties of each of the mirrors and the lens given in the options. The one whose properties match with the requirement of the spy hole in the doors will be the answer to this question.

Complete step-by-step solution:
A spy hole, also known more commonly as a magic eye, is used as an opening in the doors of apartments and the hotel rooms for looking out from the inside without even opening the door. It is used as a safety device and also for ensuring privacy of the individuals residing inside.
Now, as is clear from the requirement of the spy hole, a person is needed to look out from the inside. So we need a transparent device, or a refracting device. So we need a lens for this purpose. We cannot use a mirror for this since it is a reflecting instrument which cannot enable a person to look outside from the inside.
Therefore, both the option A and B are incorrect.
Now, for looking outside, we need a lens which covers a wide range of the area in the image. Wide range is possible if the incident light diverged after refraction.
We know that a convex lens converges the light rays incident on it. So it will form a diminished image of the outside area which will not be clear to a person looking from the inside.
Therefore, the option D is incorrect.
But we know that a concave lens diverges the incident light rays. So it forms a magnified image of the outside area, due to which more details are covered along with being clear to the person looking from inside.

Hence, the correct answer is option C.

Note: A spy hole enables us to look out from the inside. But it can also allow an outsider to look inside from the outside. This is the limitation of the spy hole. To overcome this, fisheye lenses are used.