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Science

Your senses and nervous system

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Vision defects - higher

Accommodation

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.

How the shape of the lens is changed

objectciliary musclessuspensory ligamentsshape of lens
distantrelaxstretchthin – less convex
nearcontractslackenfat – more convex

The animation shows how this works:

Short-sightedness

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.

the focus is in front of the retina

Short-sighted

lens in front of the eye brings the focus to the retina

Concave lens corrects it

Long-sightedness

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.

the focus is behind the retina

long-sighted

lens in front of the eye brings the focus to the retina

Convex lens corrects it

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