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Accommodation Accommodation is the process of focusing the eye on objects of varying distance. If light passes from one medium to another with a different optical density, the light rays are bent. In our eye this occurs first when light passes from the air through the cornea, behind which we have the aqueous humour and secondly when it passes from the aqueous humour into the lens and then again when it leaves the lens for the vitreous humour. To obtain a sharp image on the retina, light rays reflected from a point on an abject have to be focused on a point on the retina. As light rays from objects at different distances fall into our eyes from different angles, we have to change the shape of our lens to focus them on the retina (fish do this by shifting the position of the lens instead). The relaxed eye is focused on far objects. The ciliary muscle is a ring attached to the choroid1 (a layer containing blood vessels supplying the retina) and the sclera2. When it is relaxed, it is pulled outwards by the structures it is attached to, forming a large ring. The lens is attached to the ciliary muscle by the suspensory ligaments3. When the ciliary muscle is relaxed, these fibres are pulled outwards and therefore the lens is pulled into a flat shape. Light rays are bent less strongly. Those from faraway objects are focused on the retina, those from objects nearby do not form a sharp picture. To focus on closer objects, the ciliary muscle contracts, forming a smaller ring. This loosens the suspensory ligaments, permitting the elastic lens to take up a more rounded shape. Light rays are now bent more strongly, those from objects close by are focused on the retina, those from far away do not form a sharp picture. This mechanism depends on the elasticity of the lens. In older people the lens becomes stiffer and will no longer take on the rounded shape. We therefore cannot focus on things close by anymore. At the age of seventy, the lens has usually completely lost its elasticity and accommodation is no longer possible. 1 Aderhaut 2 Lederhaut 3 Ziliarfasern, Zonulafasern Do You Need Spectacles? In short-sighted people the eyeball is too long. The almost parallel rays from faraway objects meet before the retina. Nearby objects can be focused onto the retina by accommodation. The rays from faraway objects are focused onto the retina by spectacles with diverging lenses (biconcave), which bend the rays outwards, so that the eye's lens can focus them on the retina. In far-sighted people the eyeball is too short. Parallel light rays meet behind the retina. These people have to accommodate even to see faraway objects. Nearby objects cannot be seen sharply. In this case we need converging lenses (biconvex) to bend the light rays inwards.
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