To answer this question, we first need to start with the principle of refraction in the human eye. After light enters the human eye, it must pass through the cornea, aqueous humor, lens, and vitreous humor for refraction before forming an image on the retina. When viewing objects within 6 meters, the human eye primarily adjusts the convexity of the lens to make the image on the retina clearer.
Myopia or hyperopia are relatively common refractive abnormalities of the human eye. The main cause of myopia is an excessively long anterior-posterior diameter of the eyeball. Even after the human eye has fully adjusted, the image still falls in front of the retina, resulting in an unclear image on the retina. Conversely, the main cause of hyperopia is an excessively short anterior-posterior diameter of the eyeball. Even after the human eye has fully adjusted, the image still falls behind the retina, also causing visual blurriness.
So, why are all newborns farsighted? Originally, the human eye is smaller at birth, and the anterior-posterior diameter of the eye is naturally shorter, typically only 16 mm, while the anterior-posterior diameter of an adult's eye is 24 mm. Because of this, the incoming light will focus behind the retina, making everyone born farsighted. The degree of hyperopia will gradually decrease with the development of the eye. Generally, children are farsighted at about 300 degrees (equivalent to +3 meters-1) at age 4, 200 degrees (equivalent to +2 meters-1) at age 5-6, and 150 degrees (equivalent to +1.5 meters-1) at age 8. It is normal for children under 13 to be farsighted. The anterior-posterior diameter of the human eye does not reach the adult length until age 18, meaning that people completely get rid of hyperopia at this age. In preschool years, most children do not engage in long-term focused learning activities, and hyperopia does not affect vision, so it is not easy to detect in children and does not cause symptoms such as low vision, eye fatigue, headaches, and visual fatigue.