Question

Which of the following is not a proper use of periscope?
A) A periscope is used by scientists to see planetary bodies.
B) A periscope is used in submarines to see objects on the surface of the water.
C) A periscope is used by soldiers present in the trench to observe the movement of the enemy.
D) A periscope is used to observe football matches beyond the boundary wall of the playground.

Answer 1

Hint: To answer this question, one should consider the structure of the periscope and the principle on which it works. Now, observe each of the options and try to figure out if periscopes can be used for the given situations. Analyze the type of substances used in the making of periscopes and how they affect the formation of images placed at different distances.

Complete answer:
The periscope is an optical instrument used to observe objects which are prevented by direct line of sight, i.e. when our sight is obstructed by some particle or object, a periscope is used to get the image of the object which can’t be observed directly. It uses the concept of reflection of light to form the image. The mirrors placed inside it at an angle of ${45^\circ }$ can be seen in the below periscope image.

When light falls on one of the mirrors, it gets reflected and falls on the other mirror and hence reaches the eyes of the observer.
Among the given options, planetary objects are very far from our line of sight, i.e. they cannot be observed with naked eyes and need specialized instruments to observe them. A telescope is an instrument that is used to see planetary objects. As telescopes are made up of lenses, they used the refraction of light to form the images, Also, lenses can magnify the image of the planetary objects and thus giving a nice visualization of the object. Periscope uses mirrors whose magnification is one and thus cannot be used to observe planetary objects.
Therefore, option (A) is correct.

Note:
It is important to place the two mirrors at ${45^\circ }$ inside the periscope because only at this angle does light reflect away from the mirror at the same angle that it hits it. Also, the mirrors are placed at a maximum distance to avoid the formation of multiple images. The image formed by the first mirror is used as an object for the second mirror. As the mirrors are parallel, light from the first mirror gets incident on the second mirror at the right angle and the same image is formed.