what is the function of Periscope in a submarine

Dear Student,

The angle of inclination of a mirror in the periscope is inclined at 45° so that the light-ray always travel in parallel direction with the tube of the periscope. When the viewer looks into periscope, he can easily  analyse the picture outside, as the light-rays are parallel to horizontal as it would have been in normal viewing.

Because of these properties a periscope can be easily used at a concealed position like a submarine.A sailor inside can easily see outside using a periscope, even if the submarine is actually below the surface of water.