Page 59 - Biology_Form_3

Coordination in animals

Defects of the mammalian eyes However, there are other defects of the
eye which include astigmatism, presbyopia
Task 2.14
and cataract.
Search from the library and internet
sources for animations about defects Short sightedness (myopia)
FOR ONLINE READING ONLY
of the mammalian eyes and their This is a defect of vision in which far objects
corrections. Watch the videos and with appear blurred but near objects are seen
the aid of relevant diagrams write short clearly. The image of an object is focused in
notes about your observations.
front of the retina in vitreous humour rather

These are functional and structural than on the retina as seen in Figure 2.23.
deviations of the eye that alter the focusing This occurs because the refractive power
mechanism of an eye. The common defects of the eye lens is too strong or the eyeball
of the eyes are short sightedness (myopia) being too long which result in an increased
and long sightedness (hypermetropia). distance between the lens and retina.

Retina

An image formed in front
of the retina
Parallel light rays Eye lens

Figure 2.23: Short sightedness

This defect can be corrected by using a suitable concave spectacle lenses, which diverge
the light rays entering the eyes to the correct extent and bring them into focus on the
retina as seen in Figure 2.24.

Parallel light rays Diverged rays An image formed

on retina,

Eye lens
Biconcave lens
Figure 2.24: Short sightedness correction by concave lens

Long sightedness (hypermetropia)
This is a defect in which near object appear blurred but far objects can be seen clearly.
The image is focused behind the retina rather than upon it as shown in Figure 2.25.

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