Question

Describe the working of a periscope with the help of a suitable diagram. Give some uses of periscope.

Accepted Answer

An optical instrument (or device) in which reflected light is reflected again is a periscope. A periscope is a long, tubular device through which a person can see objects that are out of the direct line of sight. A periscope gives us a higher view than normal. For example, by using a periscope, we can see the objects on the other side of a high wall that cannot be seen by us directly. The periscope makes use of two plane mirrors to see over the top of things. Actually, a periscope works on the reflection of light from two plane mirrors arranged parallel to one another. A periscope consists of a long tube $T$ having two plane mirrors $M_{1}$ and $M_{2}$ fitted at its two ends as shown in the following figure.

The two plane mirrors are fitted in such a way that they are parallel to one another and their reflecting surfaces face each other. Each plane mirror, however, makes an angle of $45 °$ with the side of the tube as shown in the figure. There are two holes in the periscope tube: one hole is in front of the top mirror $M_{1}$ and the other hole is in front of the bottom mirror $M_{2}$ .

Suppose there is a tree behind a high wall which we cannot see directly. We can, however, see this tree by using a periscope. This can be done as follows: The upper hole of the periscope is turned towards the object to be seen (here a tree) so that the top mirror $M_{1}$ faces the object. We look into the periscope from the bottom hole in front of the lower mirror $M_{2}$ . The light rays coming from the tree fall on the plane mirror $M_{1}$ . The mirror $M_{1}$ reflects these rays of light downwards, towards the second mirror $M_{2}$ as indicated in the figure. The mirror $M_{2}$ then reflects the reflected rays of light towards the eye of the person looking into the periscope through the lower hole. Since the light rays coming from the tree enter the eye of the person (or observer), it is possible to see the image of tree through the periscope (even though the tree cannot be seen directly). In a periscope, the top mirror reflects light, and the bottom mirror reflects the 'reflected light'. So, the working of a periscope demonstrates that reflected light can be reflected again. The image formed in a periscope is virtual as it cannot be captured on the screen. The image is obtained in a periscope without lateral inversion. This is because two lateral inversions produced by two plane mirrors cancel each other. The working of a periscope also explains how reflection from two plane mirrors enables us to see objects which are not visible directly.

Uses of Periscopes:

(i) A periscope is used to see over the heads of a crowd (say, as in a football match).

(ii) A periscope is used by soldiers sitting in a trench (or bunker) to observe the enemy activities outside (over the ground).

(iii) A periscope is used by a navy officer sitting in a submarine to see ships over the surface of the water in the sea (even though the submarine itself may be submerged underwater).

(iv) A periscope is used in a nuclear reactor to observe the chemical reactions that are taking place.

Describe the working of a periscope with the help of a suitable diagram. Give some uses of periscope.

Important Questions on When Light Reflects

Learn and Prepare for any exam you want !