What is a periscope? How many mirrors are there in a periscope?

A periscope is a device that reflects the reflected ray again. It consists of a tube attached to a set of mirrors. There are 2 mirrors in a periscope.

The light from the object falls on one mirror that is placed at 45^o to the object and the light gets reflected. This reflected light in turn falls on another mirror and is again reflected until it reaches the eyepiece.