A periscope lets you see over the top of things, such as fences or walls that you aren’t tall enough to look over. You can also use it to see around corners.
People first started using periscopes in submarines in about 1860, to allow the sailors to see above the water. Later, soldiers in the First World War used them to look out of the trenches without having to put their heads out of the trench. Periscopes are still used today in tanks and some submarines.
A simple periscope is just a long tube with a mirror at each end. The mirrors are fitted into each end of the tube at an angle of exactly 45 degrees (45°) so that they face each other.
In the periscope, light hits the top mirror at 45° and reflects away at the same angle. The light then bounces down to the bottom mirror. When that reflected light hits the second mirror it is reflected again at 45°, right into your eye. You can see this in the picture on the right.
Light is always reflected away from a mirror at the same angle that it hits the mirror.