Question

What is Periscope and what for it?
and how to make it?

Answer 1

A periscope is an optical instrument that uses a system of prisms, lenses or mirrors to reflect images through a tube.
Cut a hole on one side just big enough for a mirror. Place one of the mirrors on one of the vertical sides of the milk carton, about 1/4 of an inch (6 mm) from the end. Trace the mirror with a pencil, then cut on the pencil marks to create a hole.
hope it helps you

Answer 2

I actually have my own periscope at home and it is available in Amazon. It is diy process and they will give you steps.
Hope this helps