Your senses and nervous system
Light is focused onto the retina by refraction at the cornea, and by the lens. The lens changes shape to make the fine adjustments needed to produce a sharp image. This is called accommodation. As we get older, accommodation becomes slower and weaker.
|object||ciliary muscles||suspensory ligaments||shape of lens|
|distant||relax||stretch||thin – less convex|
|near||contract||slacken||fat – more convex|
The animation shows how this works:
Someone with short-sightedness can see near objects clearly, but cannot focus properly on distant objects. This is caused by the eyeball being elongated, so that the distance between the lens and the retina is too great. It can be corrected by placing a concave lens in front of the eye.
Someone with long-sightedness can see distant objects clearly, but cannot focus properly on near objects. This is because the lens focuses the sharpest image behind the retina, instead of on it. This defect is often age-related, and due to a loss of elasticity in the lens. It is corrected by putting a convex lens in front of the eye.
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