Here's how it works:

* Light travels in straight lines. When light hits a mirror, it bounces off at the same angle it hit, but in the opposite direction. This is the law of reflection.

* A periscope uses two mirrors placed at 45-degree angles. The first mirror reflects the light coming in from below, directing it upwards. The second mirror reflects the light coming in from above, directing it horizontally to the viewer's eye.

This process essentially "bends" the light path, allowing the viewer to see objects that are otherwise hidden from direct view, such as objects over a wall or below the surface of water.