What exactly occurs in the eyes to make some people short-sighted and others long-sighted?
Joe Oldaker, Nuneaton, Warwickshire, UK
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Short-sightedness (myopia) and long-sightedness (hyperopia) are usually due to a mismatch between the length of the eye, from front to back, and the combined power of the cornea 鈥搕he transparent surface layer at the front of the eye 鈥揳nd the eye鈥檚 crystalline lens to converge rays of incoming light.
Light rays entering the eye are bent by the cornea and lens to converge at a focal point, which should be on the retina at the back of the eye to make a sharp image. In myopia, rays from far-off objects focus in front of the retina and the image is blurred. In hyperopia, nearby objects focus behind the retina, with a similar result.
The excessive convergence of rays in myopia and the reduced convergence in hyperopia can be corrected by appropriate lenses worn in front of the eye. Laser surgery to reshape the cornea is also used, mainly for myopia.
An additional complication is that the lens in the eye is flexible, and can, via muscular control, change shape to add convergence power. This is automatic when looking at things that are near, such as words in a magazine, so they can be seen clearly. This effect can mask some long-sightedness.
鈥淐hicks raised in red light grow up to have longer eyeballs and become short-sighted鈥�
In later life, the eye lens becomes less flexible and this ability fades, so reading glasses are needed to see close objects sharply. However, people with myopia have extra convergence built in and so can often do without the reading glasses most of us need with age.
Mike Follows, Sutton Coldfield, West Midlands, UK
Lifestyle rather than genetics seems to account for the increased prevalence of myopia seen in recent times.
This idea came to prominence with a published in 1975, entitled 鈥�Inuit myopia: an environmentally induced 鈥榚pidemic鈥�?鈥�, that compared the eyesight of younger and older Inuit people. It showed that about 31 per cent of those below 30 years of age were myopic compared with about 4 per cent of their parents. The younger generation had spent much more time in school.
Many subsequent studies have shown that time spent outdoors makes it less likely that people become short-sighted. This , which stimulates production of the signalling molecule dopamine, and so influences the development of the eyeball.
Another even more convincing , led by Chi Luu at Melbourne University in Australia, suggests that the colour of light is the key. The cornea does the lion鈥檚 share of the focusing while the lens does the fine-tuning.
Like glass, the cornea refracts red light less than blue light, so red light comes to a focus further behind the cornea. The team found that chicks raised in red light grow up to have longer eyeballs and become short-sighted, while those raised in blue light have shorter eyeballs.
Given that we now tend to spend more time indoors, where light is usually redder, it is possible that our eyes have adapted accordingly and short-sightedness is the consequence. Getting children playing outside, where they are exposed to more blue light, may reduce their risk of becoming short-sighted. Alternatively, we could make indoor lighting mimic daylight.
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