It isn't entirely worked out what cues the eye that it is time to stop growing, but we know that in many kids the eye continues to grow past emmetropia and they become nearsighted.
Basically, when the eye grows too long the light inside the eye comes to a focus in front of the retina rather than at the retina, causing blurry vision, so we must wear glasses to change the optics and focus the light onto the retina again.
Many older folks must wear bifocals which have two different lenses- one to correct for the problems with near vision and one to correct for problems with far vision.
More than half the young children and teenagers in China are nearsighted, according to a survey by top government agencies, which called for intensified efforts to prevent and control the condition.
The survey, which was conducted last year, found that eight of 10 senior middle school students were nearsighted, compared with 71.6 percent in junior middle school, 36 percent in primary school and 14 percent of 6-year-olds in kindergarten. Overall, 53.6 percent were nearsighted.
The prevalence of a high degree of myopia also became alarming as the percentage of senior students in high school, who wear glasses stronger than six diopters, has mounted to 21.9 percent.
Up to 80 percent of the country’s young adults suffer from nearsightedness, according to a report in the medical journal Lancet. In contrast, the overall rate of myopia in the UK is about 20-30 percent.
If you walk the streets of China today, you'll quickly notice that most young people wear glasses. In Shanghai, for instance, 86 percent of high school students suffer from myopia, or nearsightedness, according to Xinhua News Agency.
The growing prevalence of myopia is not only a Chinese problem, but it is an especially East Asian one. According to a study published in The Lancet medical journal in 2012, by Ian Morgan, of the Australian National University, South Korea leads the pack, with 96 percent of young adults (below the age 20) having myopia; and the rate for Seoul is even higher. In Singapore, the figure is 82 percent.
ZME Science的标题更是吓人:To say that Asia is having an eye problem is an understatement.说是“亚洲近视”都不为过。
近视的原因到底是什么?
China Daily的报道总结了近视的几大原因:
Several factors are associated with the high rate of nearsightedness in China's children and teenagers, including lack of outdoor physical activity, lack of adequate sleep due to heavy extracurricular work and excessive use of electronics products.
Biologists compared Singaporeans living in Singapore to those living in Australia. They found that 29 percent of the Singaporean students had myopia compared with just 3 percent in Sydney. The main correlation was once again, time spent outside.
“The big difference was the Chinese children in Australia were outdoors a lot more than their matched peers in Singapore,” says Ian Morgan, a retired biologist at Australian National University, who coauthored the 2008 study. “This was the only thing that fit with the huge difference in prevalence.”