††A periscope is a device with mirrors for getting a view as if from a higher position.The periscope was used widely in the First World War to allow submerged submarines to see above the water, soldiers to see over trenches, and surgeons to see inside the body.
Today, submarines are still fitted with periscopes, while smaller versions allow people at the back of a crowd to see what is happening at the front.
In the simplest version, two mirrors are set at angles of 45 degrees within an upright tube. If their reflective surfaces are opposite one another, the light rays entering the tube horizontally at the top are turned through 90 degrees by each mirror. This means that they are directed downwards by the first mirror, before the second returns them to their original direction. Thus the observer may be sighted some distance below the viewing lens.
