Analyze the security door "cat eyes"

The filler in the security door should be able to play the role of sound insulation, warmth and fire prevention. Therefore, the state's internal filling requirements for the security door must be aluminum silicate rock wool, because the aluminum silicate rock wool has good insulation, fireproof and soundproofing. performance. Like the telescope and the microscope, the cat's eye is composed of the objective lens and the eyepiece. It is known from the "self-learning 2" that the telescope is composed of two convex lenses, the objective lens has a long focal length, and the eyepiece has a short focal length; the microscope is also composed of two convex lenses. The focal length of the objective lens is very short and the focal length of the eyepiece is long.

The security door "cat's eye" cannot be composed of two convex lenses, because the thickness of the door is about 4cm, and the distance between the objective lens and the eyepiece is about 3cm, which also determines that the focal length of the objective lens and the eyepiece cannot be long, otherwise it will not be satisfied. The effect of the cat's eye", then the structure of the "cat's eye" security door "cat's eye" is composed of two concave lenses. Can it be satisfied by the imaging law that the object through the objective lens (concave lens) is reduced to the erect virtual image, the virtual image through the eyepiece (concave lens) ) The reduced erect virtual image, the distance from the virtual image to the eye is less than the distance between the eyes, and the hole of the "cat's eye" is only about 1cm, and the concave lens diverges. The light entering the human eye is also weak, that is, it cannot Look at the scenery, its light path diagram is shown in Figure 1 (the scene AB is far from the objective lens is not drawn).

Therefore, the "cat's eye" is not composed of two concave lenses. The security door "cat's eye" is composed of a convex lens and a concave lens to meet the requirements. When the objective lens is a convex lens and the eyepiece is a concave lens, it can be known from the lens imaging law that the scene (1 to 2 m from the door) is reduced to an inverted real image by the objective lens (convex lens), and the image is used as a virtual object of the eyepiece (concave lens) to form a reduced virtual image. If the human eye is close to the eyepiece (concave lens), this distance is much smaller than the apparent distance and cannot be seen clearly.

If the person observes away from the eyepiece (concave lens), the imaged light of the scene enters the human eye too little, and the angle of view is also small, and it is also impossible to see. It is concluded that the use of convex lens crop mirrors and concave lenses as eyepieces is not satisfactory. But it also gives us inspiration, it just meets the requirements of not being able to see the indoor scene outdoors.