The Human Eye: Function (Edexcel IGCSE Biology)
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The Human Eye: Function
The function of the eye in focusing on near and distant objects
The way the lens brings about fine focusing is called accommodation
The lens is elastic and its shape can be changed when the suspensory ligaments attached to it become tight or loose
The changes are brought about by the contraction or relaxation of the ciliary muscles
When an object is close up:
The ciliary muscles contract (the ring of muscle decreases in diameter)
This causes the suspensory ligaments to loosen
This stops the suspensory ligaments from pulling on the lens, which allows the lens to become fatter
Light is refracted more
Diagram showing the eye when an object is close up
When an object is far away:
The ciliary muscles relax (the ring of muscle increases in diameter)
This causes the suspensory ligaments to tighten
The suspensory ligaments pull on the lens, causing it to become thinner
Light is refracted less
Diagram showing the eye when an object is far away
Focusing on distant and near objects table
| Near objects | Distant objects |
---|---|---|
Ciliary muscles | Contracted | Relaxed |
Suspensory ligaments | Loose | Tight |
Lens | Fatter | Thinner |
Light refraction | More | Less |
Examiner Tips and Tricks
A common mistake is to refer to the suspensory ligaments contracting and relaxing. This is incorrect because suspensory ligaments are not muscles. You must say that the ligaments tighten or loosen/slacken instead.
The function of the eye in responding to changes in light intensity
The pupil reflex is a reflex action carried out to protect the retina from damage
In dim light, the pupil dilates (widens) in order to allow as much light into the eye as possible to improve vision
In bright light, the pupil constricts (narrows) in order to prevent too much light from entering the eye and damaging the retina
The pupil reflex
The pupil reflex in dim light
The pupil reflex in bright light
Pupil reflex table
Stimulus | Radial muscles | Circular muscles | Pupil size | Light entering eye |
---|---|---|---|---|
Bright light | Relaxed | Contracted | Narrow | Less |
Dim light | Contracted | Relaxed | Wide | More |
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