What is the principle of a door viewer？

The design of the door viewer allows it to see images with good imaging quality from the inside out, while it is difficult to see the image from the outside in. Therefore, the cat's eye looks very clear from the inside and blurs from the outside.

Door viewer, also known as a door mirror, is a small optical instrument installed on a residential door. Its function is to allow people inside the house to observe the outside and ensure that those outside cannot see clearly inside the house. 

A simple door viewer consists of two lenses, one concave lens on the outside side, and the other convex lens on the inside side. The concave lens has a very short focal length, while the convex lens has a longer focal length.