Question

a periscope is a useful device as every one agree it has which mirrors fixed inside at - degree.

Answer 1

90° is your answer.

Hope it helps :)

Answer 2

A periscope is an optical instrument that uses a system of prisms, lenses or mirrors to reflect images through a tube. Light from a distant object strikes the top mirror and is then reflected at an angle of 90 degrees down the periscope tube.