Use the principle of lenses to talk about how myopia is formed.

Myopia is caused by looking at nearby things for a long time. The lens in the eye is equivalent to a convex lens. If you keep looking at nearby things, the lens will shrink (that is, become thicker) for a long time. Because the refractive power of the lens becomes stronger when it becomes thicker. If this happens for a long time, the lens will continue to become thicker and it is difficult to thin. Because the refractive power becomes stronger, the focus of distant light after refraction of the lens cannot be It happens to fall on the retina, but falls in front of the retina, so you cannot see clearly, which is myopia.