Concave lens use – Peephole or door viewer or spy hole
Many doors in hotels have a peephole in them which allows guests to see who's outside the door. People outside the door can not see in. One of the most popular designs is to use three concave lenses and one convex lens as follows:
| 1, 2 and 3 = | Concave lenses | |
| 4 = | Convex lens |
The ray diagram for a peephole shows how light from a very wide angle can provide a clear view of a person through a small aperture.
If somebody tries to see from the outside in, their eye can capture only a small range of angles and only see a magnified view of a small part of the light that enters from the inside of the room.
