Page 60 - Biology_Form_3

Biology for Secondary Schools

This occurs when the refractive power of the eye’s lens is too weak or when the eyeball
is too short which result in shortening of the normal distance between lens and retina.

Divergent light rays
Retina
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An image formed
Eye lens behind the retina

Figure 2.25: Long sightedness defect

This condition can be corrected by the use of a suitable convex spectacle lens, which
converges the light rays entering the eye to the correct extent and brings them into focus
on the retina as shown in Figure 2.26.

Retina

An image formed
Convex lens Eye lens on retina

Figure 2.26: Long sightedness defect correction by convex lens

Astigmatism Cataract
This defect occurs when the eye does not Cataract is another eye defect that happens
focus light evenly on the retina, instead when the lens become opaque and will not
some light is focused on the retina while let light through at all. This can be corrected
some is focused behind or in front of through surgical removal and replacement
the retina. This is usually caused by a of the lens with an artificial one.
non-uniform curvature of the cornea. A
typical symptom of astigmatism is if you Presbyopia
are looking at a pattern of lines placed at Presbyopia is an eye defect related to aging
various angles, the lines running in one process. It is caused by gradual thickening
direction appear sharp whilst those in other and the loss of natural lens flexibility.
directions appear blurred. Astigmatism can Presbyopia occurs mostly to people over
usually be corrected by using a special age of 40 years. The changes occur within
spherical cylindrical lens. This is placed the proteins in the lens by making the lens
in the out-of-focus axis. harder and less elastic over time. The

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