What is the hole you look through on a door called?

peephole

A peephole is a small hole in a door or wall through which you can look secretly at what is happening on the other side.

How do I install a door scope?

Quote from video: Then tighten the screw with your hand then insert the bottom mounting screw and washer tighten both screws with a screwdriver the installation is almost complete insert the door viewer.

How does a door spy hole work?

Wide angle lenses capture more light than a flat lens because light coming from the sides can still impact it and be refracted. The peephole lens then condenses all of the light it collects down into a tiny stream that is then fed into the eye of whoever is looking through it.