China has an ancient book "Ten Thousand Books in Huainan", which records: "Take a big mirror, hang it high, put a pot under it, and you will see your neighbors ..." This is used.

This is to use the imaging characteristics of flat mirror to observe the surrounding things, and its device is similar to the modern periscope device. Therefore, it is the prototype of periscope in modern military. Plane mirror imaging applies the principle of light reflection.

Therefore, the answer is: flat mirror imaging; periscope