Accomodation by MikeJenny

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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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