The brief introduction to door viewers

A door viewer is a small optical instrument installed on the door of the residence, which is convenient for residents to observe the outdoor situation. It has become a necessity for home security. It consists of a concave lens (objective) and a convex lens. The focal length of the objective lens is extremely short, which makes the outdoor person or object form an upright and reduced virtual image, which falls within the focus of the eyepiece, and finally obtains a reduced upright virtual image, which is just at the nearby distance of the human eye, the situation outside the door can be seen clearly. Looking out through the door viewer from the inside, you can see all the scenes within the range of about 120 degrees outside the door, but from the outside through the door viewer, you can't see anything in the room. If this door viewer is installed on the gate of public houses or private apartments, it can play a certain role in the security and safety of the family.