A bifocal mirror is a lens with two focal points. Originally invented by Benjamin Franklin. At that time, the glasses he wore had only one focus. Looking at the near and distant switching, it would cause eye fatigue. Later, he split the lens into two, and the two upper and lower lenses with different focal lengths were combined to form a new lens. The upper half is suitable for looking far away and the lower half is suitable for looking close. Due to the imaging principle of the lens, even a half-piece lens can still present a complete image, and the two images do not interfere with each other. In the past, whether it was myopia or reading glasses, it was called a single-focus mirror, and there was only one focus. Later developed into a double-focus mirror, the same lens is divided into upper and lower parts. When you look far, use the upper part of the lens and look down when you look at it. It is just our reading distance, and naturally the lower part is used. If there is 400 degrees of myopia, the upper part is 400 degrees, and the distance is clear. The following part loses 150-200 degrees of myopia, leaving 200 degrees of myopia.
A large number of experiments have shown that this degree can be accepted by children. In order to prevent myopia, people who face the eye can also use, the upper part is 0 degrees, that is, there is no myopia, used to look far. In the lower part, there are 150 degrees to 200 degrees of farsightedness, which can be used for reading and writing. At present, some glasses that protect eyesight and prevent myopia are mainly double-focus lenses.