A door viewer, also known as a peephole, is a small optical device installed in a door that allows a person inside to see who is outside without opening the door. Typically made of glass or plastic, it provides a wide-angle view, enabling homeowners to identify visitors or potential intruders safely. Door viewers are commonly used in residential and commercial buildings for added security. They come in various designs and sizes, and some modern versions even include digital cameras that can record video or send images to a smartphone, enhancing safety and convenience for users.