As the picture shows, it is a phenomenon recorded in the ancient book Huainan Wanbi Shu: "Take a big mirror and put a pot under it, and you will see your neighbors ..." This is the use of light.

The water surface of the basin is equivalent to a flat mirror. "Hang a big mirror on it and you can see your neighbors." This is to observe the surrounding things by using the imaging characteristics of a flat mirror, which belongs to the principle of light reflection.

Its device is similar to the modern periscope device.

So the answer is: reflection; periscope