Why is the door mirror called "cat's eye"

2019-12-14 10:18:40 admin

The real name of the cat's eye is the door mirror. Why is it called the cat's eye? Because the light refracts through it and looks at it in the distance. It looks like the light announced by the Persian cat's eye at night, so it's called the cat's eye

The effect and principle of cat's eye

I. Effect of door mirror

Looking out from the inside through the door mirror, you can see all the phenomena within the 120 degree field angle outside the door, but you can't see anything inside from the outside through the door mirror. If you install this mirror on the door of a public house or private apartment, you can have a certain effect on the security and safety of the family


II. Optical principle of door mirror imaging

The door mirror is composed of two lenses. When we look from the inside of the door to the outside, the objective lens L1 is a concave lens, and the objective lens L2 is a convex lens (see Fig. 1 for the optical path). The focal length of the objective lens L1 is very short, which shrinks the outdoor person or object AB into a very small upright virtual image a ′ B ', which just falls in the top focus of the objective lens L2. L2 plays the effect of expanding the lens, and finally gets a relatively enlarged one The upright virtual image a ″ B ″, which is just next to the visual interval of the human eye, can be seen clearly about the situation outside the door

Then, why can't you see anything when you look inside (backward) from the outside of the door after passing through the door mirror?

When looking backward (see Fig. 2 for optical path), L1 becomes an eyepiece, and L2 becomes an objective lens. The indoor scene AB, the refracted light beam after the converging lens L2 should have generated an inverted real image a ′ B ', but before imaging, it falls on the divergent lens L1. Because the focal length of L1 is very short, the final upright virtual image a ″ B ″ is very close to the eyepiece L1, only 2-3cm, and the hole of the door mirror The diameter is very small, and people outside have to close to the eyepiece L1 to observe. In this way, the distance between the human eye and the image a ″ B ″ is only 2-3cm, which is far less than the near point of the normal human eye. Therefore, for the outdoor peepers, all of the indoor is "blind" (the near point is the minimum distance between the human eye and the object. The near point distance increases with the age The proximal point of normal young people is about 10 cm, but the proximal point of normal people is about 40 cm when they are 50 years old

Article source: anti theft door accessories manufacturerhttp://www.xh-mingyuan.com

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