A periscope is an optical instrument that uses a system of prisms, lenses or mirrors to reflect images through a tube. Light from a distant object strikes the top mirror and is then reflected at an angle of 90 degrees down the periscope tube A periscope works on the Laws of Reflection which states that the light from the object falls on one mirror at a 45 o angle from the object and is reflected. This reflected light then falls on another mirror and is again reflected until it reaches the human eye
Note : You will notice there is 6-inch-square openings at the top front and lower back of the chamber.
Look into one hole and adjust. Do you see a clear image when you look through one of the holes? If it is blurry, or you only see the inside of the periscope, adjust the position of the mirrors. Once both of them are at 45º angles, you should be able to see through the periscope clearly. In a periscope, light from an object strikes the top mirror at 45° and bounces off at the same angle. This sends light directly down the tube and onto the lower mirror. This mirror, also at a
45° angle, reflects light directly to your eye.
To understand the optics of periscopes, think of the letter Z. The light reflected from an object hits a mirror tilted at45 degrees at the top of the device. The light then bounces to an identically angled mirror at the bottom, providing the image we see when we look into a periscope. If you follow the path of light, it's approximately Z-shaped.