The eye

• Accommodation is a reflex that changes the refractive power of the lens so that you can see both near and distant objects.
• Glasses can help when the accommodation process doesn't work properly.
• The cornea and the lens both refract or bend light.
• The ciliary muscles and suspensory ligaments control the shape of the lens.
• When light from an object hits the eye, it has to be focused onto a spot of the retina called the fovea.
• The cornea always attracts light by the same amount.
• The role of the lens is to fine-tune the refraction so that the light rays always converge exactly on the fovea regardless of whether the object is really far away or very close.
• If an object is close to us, the light from its surface has to be refracted a lot which means that even once it's been refracted by the cornea it still needs a powerful lens to refract it some more.
• The lens needs to be short and fat so that it's more curved which means that it can refract more strongly.
• Whenever you look at a nearby object, the ciliary muscle contracts and it actually contracts inwards towards the lens because it's now closer to the lens.
• The suspensory ligaments slacken which means they become loose and because they're no longer pulling tight on the lens the lens is free to return to its natural fatter shape so it can now refract light more strongly.
• For distant objects, the light doesn't need to be refracted as strongly which means that the lens doesn't need to do as much because the cornea has already refracted the light most of the way to reduce the refractive power of the lens.
• To achieve this, the ciliary muscle relaxes moving away from the lens and pulling these expensive ligaments taut which stretches the lens out.
• This allows light rays from far away objects to be focused onto the retina.
• Long-sightedness, also known as hyperopia, results in people being unable to focus on nearby objects because the lens can't refract enough.
• When we look at distant objects, the ciliary muscles relax, causing the suspensory ligaments to pull on the lens, making it flatter.
• Short-sightedness, also known as myopia, results in people being unable to focus on distant objects because the lens refracts light too much.
• The lens is held in place by the suspensory ligaments, which are attached to the ciliary body.
• Glasses containing convex lenses help people with long-sightedness by providing some extra refracting power so that the eye is then able to focus the light onto the retina properly.
• Glasses containing concave lenses help people with short-sightedness by refracting light outwards and so counteract the over-refraction of the lens which allows the light to focus on the retina like we want.
• The medical name for long-sightedness is hyperopia and the medical name for short-sightedness is myopia.
• Short-sightedness, also called myopia, occurs when an individual can see nearer objects clearly but distant ones appear blurred.
• So what happens is that the light comes through the lens and focuses behind the retina instead of directly onto it.
• As a result, images are formed behind the retina instead of directly on it.
• In short sighted individuals, the eyeball may be longer than normal or the cornea steeper than normal, resulting in the image forming in front of the retina instead of directly on it.
• When we're looking at an object near us, our lenses have to bend the light coming into them by quite a lot but if the lens is not very strong then it won't be able to bend the light enough.
• In long sighted individuals, the eyeball is too short or the lens is not flexible enough, meaning that even when relaxed, the lens cannot flatten sufficiently to allow light rays to converge on the retina.
• If your eyes are long sighted then the eyeball is too short or the lens isn't thick enough so when you focus on something close up the image will be blurry.
• Long-sightedness, also called hyperopia, occurs when an individual can see distant objects more easily than nearby ones.
• Astigmatism is where there is uneven curvature of the cornea or lens causing distorted vision at all distances.
• Presbyopia is where the lens loses its flexibility due to ageing making it difficult to focus on close objects.
• To correct this problem, glasses have been invented that contain concave lenses.
• In short sighted individuals, the eyeball is too long or the cornea is too curved.
• Concave lenses reduce the amount of refraction taking place as they bend the light rays back towards their original path.
• This means that the image will be fuzzy because the light rays have not converged at one point.
• In this case, the person would need glasses or contact lenses with concave lenses as they bend the light back towards the centre of the eye.