A peephole contains a lens providing anyone answering the doorbell with a 180-degree field of view, making it impossible for someone outside to be out of sight. There are two kinds, both available at hardware stores.
One is small, usually 1/2 inch in diameter: the other, 1.5 inches in diameter, has a one-way mirror so that the person outside can’t see you.
At a height that’s convenient for the shortest adult, drill a hole in the center of a wood or metal door. Use a high-speed steel twist bit as wide as the viewer shank for the small-diameter peephole; drill for the large-diameter kind with a hole saw attached to your electric drill. Screw the adjustable halves of the viewer together through the hole. Tighten with the edge of a coin.