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 and metal, it provides a wide-angle view, enabling homeowners to identify visitors or potential intruders safely. Door viewers are commonly used in residential settings for added security. They can be found in various styles and sizes, often designed to fit standard door thicknesses. Some modern versions even include features like a camera or digital display, enhancing the traditional functionality of a basic peephole.