A person who never forgets anything

Distance between long-distance visibility: 200× relative height of object (m)

A very simple eye structure can detect the light-dark boundary of the surrounding environment, and a more complex eye structure can give visual effects. Compound eyes generally exist in arthropods (such as insects) and generally consist of many simple pores, which form an image (rather than multiple images as generally thought).

People's eyes can often see distant objects clearly, because it can focus external light on their eyes. Normal eyes are all about 24mm long, so much external light must be fully focused within such a short distance (24mm), which is mainly due to the strong focusing ability of cornea. As mentioned earlier, the cornea has a focusing power of 4300 degrees.

When people's eyes change from seeing far to seeing near, they should not only adjust the light to focus on the retina, but also turn their eyes inward to make the observed object on the visual axis. The process of turning the eyes inward to make the objects fall on the visual axis is called combination.

In many mammals and some coelenterates, the eyes develop by shining light on the light-sensitive retina, where the light is received and converted into data signals and transmitted to the head according to nerve fibers. Generally speaking, the eye is spherical, full of completely transparent curing chemicals, with an eye lens for focusing and a retina that can adjust the amount of light entering the eye.

In order to produce a clear image (focus) of an object, the eyes must bend the light from the reflective surface of the object, so that the light will immediately fall on the retina. People who are nearsighted can only recognize objects at close range, while objects at a distance look blurred because the image is focused in front of the retina.

The method of correction is to let the nearsighted people wear concave lenses, which can make the focused light move back slightly and just fall on the retina. The factor of hyperopia is that the inner and outer diameters become shorter. The light emitted by a close object is focused behind the retina, so it looks blurred and can only be seen clearly by a distant object. Hyperopia patients should wear a convex lens for imaging, so that the light bends slightly inward to help the light fall on the retina.