Question

In a periscope the plane mirrors are at 45°? why?

Answer 1

It is because then the falling one the one is able to move along the length of periscope and when it falls on the next mirror then it comes out from the other end and gives us the image of the object. 

Answer 2

In a periscope the plane mirrors are at 45°
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because
1)the mirrors will be opposite to each other in such a way that whatever is seen in the first mirror will be seen through the second to a maximum extent

2) we can see the things happening above or below us when there is something blocking our vision

3) as the mirrors are  set in the parallel manner at 45 degree we can see the image more accuartely