Page 243 - Biology_F5

Biology for Advanced Level Secondary Schools

Conjunctiva Sclera

Choroid
Iris
Lens Vitreous humour Fovea
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Pupil Blind spot
Cornea Optic nerve

Suspensory Retina
ligament
Ciliary Extrinsic
muscle
body

Figure 4.27: Vertical section of the human eye
Eye accommodation muscles of the iris diaphragm contract, the
This is the reflex mechanism by which radial muscles relax, the pupil becomes
light rays from an object are brought smaller, and less light enters the eye. This
to focus on the retina. It involves two process prevents damaging the retina and
processes, namely reflex adjustment of the increases the depth of focus. In dim light,
pupil’s size and refraction of light rays. the circular muscles of the iris diaphragm

relax, the radial muscles contract, the
Reflex adjustment of the pupil’s size pupil become large, and more light enters
This involves the control of the amount of the eye. This decreases the depth of focus
light entering the eye, either bright light of the eye (Figure 4.28).
or dim light. In bright light, the circular

Dim light Circular muscle Bright light Radial muscle
relax relax

Circular muscle
contract
Radial muscle
Pupil contract
Pupil dilates Pupil constricts
(a) (b)
Figure 4.28 Reflex adjustment of the pupil’s size in (a) dim and (b) bright light

Refraction of light rays than 6 m away), are nearly parallel to
Refraction of light rays involves bending one another towards the eye while those
of light rays as they move through from a closer object tend to spread out
different media in the eye. At all distance (diverging) as they make an angle from
the object. In all these two cases, light
range, light rays enter in the eye, they are rays must be refracted or bent to focus on

refracted and they focus on the retina. the retina.
Light rays from a distant object (more

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