It is an instrument in which plane mirrors are used to fold light so that the image of an object can be brought down to a lower level. It is used for observing enemy movements from trenches without any danger of being seen. Sailors on submarines use periscopes to see things above the water level.
It consists of two parallel mirrors A and B facing each other and each fixed at 450 to the frame work. Rays of light entering through the aperture, strike the mirror A at an angle of incidence equal to 450 and are reflected along the axis of the tube, striking the mirror B at 450. From the mirror B, these rays are reflected parallel to their original path reaching the eyes of the observer.
