Question

The mirror used in periscope is:
A. Convex mirror
B. Concave mirror
C. Plane mirror
D. Any of the above can be used

Answer 1

The periscope brings the image of an object to the observer above or below, considering the rectilinear property of light, by reflecting it on mirrors twice. This is used to visualise the outside surface of a submarine. It is practically impossible for a voyage under water to swim up to the surface to check for some disturbance. From the principle of reflection this specific instrument came into picture and rescued the submarines.

Complete solution:

The image of the object does not reach the observer's eye when the object and the observer are at two different levels, as light can only travel in a straight line. At a $45^{\circ}$ inclination to the incoming object ray, the reflection from one mirror will direct the rays perpendicular to the incoming direction.

The reflection at an inclination of $45^{\circ}$ from the second mirror will cause the required level shift to reach the observer. For this purpose, plane mirrors serve well and are covered by a perpendicular Z-shaped pipe. Plane mirrors reflect the object's exact shape and size. The picture's distance is also the same as that seen from the reflection of plane mirrors. But a complicated structure of these mirrors is used to obtain the fine image for better viewing results.
Therefore, the interior of the periscope is formed by plane along with lenses. The mirror used in periscope is a plane mirror.

The correct option, therefore is C.

Note:
Only plane mirrors are used for a simple periscope. An expanded view of the object is required for some distance visions, and therefore convex mirrors are helpful. An erect image is produced by a double reversal of the object. Convex mirrors can be used to view objects that are too close to each other.