Question

The instrument that can be used to see things from a concealed position is _____________ .
A) Periscope
B) Kaleidoscope
C) Telescope
D) Microscope

Answer 1

The observer is in a concealed position. So a single mirror or a single lens or a single prism will not do the trick. A system of mirrors or lenses or prisms inclined at some angle is required. Periscope is an optical instrument which can be used to see the object around, over or through an obstacle.

Complete step by step answer.
Step 1: Sketch a figure depicting the observation of an object from a concealed position using a periscope. Try to describe the image formation.
When the direct line of sight to an object is blocked by an obstacle a periscope can be used to view the required object.
The figure given below depicts the observation of a tree (object) by an observer behind a wall using a periscope.

The periscope comprises two plane mirrors ${M_1}$ and ${M_2}$ fitted at an angle of $45^\circ$ at its two ends. The light from the object enters through one end of the periscope. The light ray gets reflected by ${M_1}$ and it is then incident on ${M_2}$. The reflected ray from ${M_2}$ then reaches the eye of the observer and the observer gets a virtual and erect image of the object.

So the correct option is A.

Note: Plane mirrors produce laterally inverted images. However, here we use two plane mirrors. The laterally inverted image formed after the reflection of the light rays from the first mirror ${M_1}$ will serve as the object for the next mirror. The once laterally inverted image then gets inverted again laterally upon reflection from the second mirror ${M_2}$. So the image reaching the observer will be virtual and erect but without lateral inversion.