In a periscope, the two plane mirrors are kept

A simple periscope is an optical instrument used to see objects that are not in the direct line of sight, commonly utilized in submarines, tanks, and bunkers [1]. It consists of a tube containing two plane mirrors fixed parallel to and facing each other. To function effectively, each mirror is typically inclined at an angle of 45° to the horizontal or the line joining them. When light from an object strikes the first mirror at a 45° angle of incidence, it is reflected at 45° (following the laws of reflection), resulting in a 90° turn of the light path. The light then travels down the tube to the second parallel mirror, where it undergoes another 90° reflection into the observer's eye. This parallel arrangement ensures the final image is erect and the light path remains consistent.

Sources

1. [1] Science-Class VII . NCERT(Revised ed 2025) > Chapter 11: Light: Shadows and Reflections > 11.8.1 Periscope > p. 164