Question

why are the two mirrors in a periscope placed parallel to each other

Answer 1

the two mirrors in a periscope placed parallel to each other so that the light coming into the periscope can reflect from first mirror to the second and from the second to our eye.

Answer 2

Generally the mirrors in a periscope are placed at an angle of 45° at a maximum distance is made so that they will not form such multiple images as both the mirrors are placed far from each other, they could only reflect the light rays and thus reflects back to our eyes, this is the reason they are placed being parallel facing to each other.

Hope this helps you!! :)