Common eye defects

 

As people get older the lens of the eye may begin to harden, so accommodation becomes more difficult and they cannot focus so easily on close objects. This can make reading a problem, for example. However, it isn’t only older people who can have problems with their vision. There are a number of common eye defects that can and do affect many people of all ages. There are a number of ways in which these problems can be treated – but they all involve the use of extra lenses to bend the light. A lens is a piece of transparent material (usually glass or plastic) that has one or more curved surfaces. An outward curve makes a convex lens and this will bend the light rays towards each other (a converging lens). An inward curve makes a concave lens and this will spread the light rays out (a diverging lens). In some people the normal method of accommodation in the eye that enables them to see close-up and distant objects with equal clarity just doesn’t work properly: Short sight: A short-sighted person can focus clearly on things that are close to them but has much more difficulty with objects in the distance, which appear blurred. This may be as a result of a lens, that is effectively ‘too strong’ – it is too curved even when the ciliary muscles are fully relaxed and so the light from distant objects is focused in front of the retina, making the image that actually lands on the retina spread out again and blurry. Another cause of short sightedness is when the lens is normal but the eyeball is particularly long – and again this means light is focused in front of the retina. This problem can be corrected using concave (diverging) lenses that spread the light out more before it gets into your eye. This means that the thicker lens can bring the rays of light into perfect focus on the retina – or there is room in the long eyeball for the light rays to be focused on the correct point

Long sight: A long-sighted person can focus clearly on things that are at a distance but has much more difficulty with objects close to them, which appear blurred. This may be as a result of a lens that is effectively ‘too weak’ – it is too flat even when the ciliary muscles are fully contracted and so the light from close objects is focused behind the retina, so the image that actually lands on the retina is spread out and blurred. Another cause of long sightedness is when the lens is normal but the eyeball is particularly short – and again this means light is focused behind the retina. This problem can be corrected using convex (converging) lenses that bring the light rays together more before they reach your eye. Now the thinner lens can bring the rays of light into perfect focus on the retina – or the short eyeball becomes the right length for the light rays to be focused on the correct point.