In a periscope, the two plane mirrors are kept parallel to each other.
Option 1: This is the correct answer. The mirrors in a periscope are kept parallel to each other to redirect light from one mirror to the other. The first mirror reflects the light from the object to the second mirror, and the second mirror then reflects the light to the viewer`s eye.
Option 2: Mirrors placed perpendicular to each other would reflect the light at a 90 degree angle, which would not work for the purpose of a periscope, as it wouldn`t guide the light to the observer`s eye.
Option 3: An angle of 60° would also not direct the light appropriately to the viewer. It would scatter some of the light away from the viewer`s eye.
Option 4: With an angle of 45° the light would be deflected off at a right angle from the direction of incidence. It wouldn`t guide the light effectively to the viewer`s eye.