The Eye (Cambridge (CIE) O Level Biology): Revision Note
Structure & Function of the Eye
The eye is a sense organ containing receptor cells that are sensitive to light (rod cells) and colour (cone cells)
The eye diagram
The eye is adapted to detect light stimuli
Function of the eye table
Structure | Function |
|---|---|
Cornea | Transparent lens that refracts (bends) light as it enters the eye |
Iris | Controls how much light enters the pupil |
Lens | Transparent disc that can change shape to focus light onto the retina |
Ciliary muscles and suspensory ligaments | Control the shape of the lens |
Retina | Contains light receptors, some sensitive to light of different colours |
Fovea | Contains the greatest density of light receptors |
Optic nerve | Sensory neurone that carries impulses to the brain |
The blind spot
At the point where the optic nerve joins the retina, there are no light-sensitive rod and cone cells on that part of the retina
Light falling onto that part of the retina will not result in an image being detected
the brain 'fills in' from surrounding light so we don't see a black hole where no light has fallen
This causes a blind spot, where we cannot detect an object in our peripheral vision even if it is there
The Pupil Reflex - Antagonistic Muscle Action
The pupil reflex
This is a reflex action carried out to protect the retina from damage in bright light and to increase our ability to see objects in dim light
In bright light the pupil constricts (narrows) in order to prevent too much light entering the eye and damaging the retina
In dim light the pupil dilates (widens) in order to allow as much light into the eye as possible
Pupil reflex diagram
The pupil reflex protects the retina in bright light and maximises light reaching the retina in low light
Pupil reflex muscles diagram
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 |
Accommodation
Accommodation: the function of the eye in focusing on near and distant objects
The way in which 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
Focusing on a near object diagram
The ciliary muscles contact and the suspensory ligaments loosen to allow the eye to focus on a near object
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
Focusing on a distant object diagram
The ciliary muscles relax and the suspensory ligaments tighten to focus on objects that are 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
The focusing of the eye on distant and near objects is complex and it can be hard to remember what is happening. This is something you can work out in an exam if you have forgotten – staring at your hand right in front of your eye will make your eyes feel tight and tired after a few seconds. This is because the ciliary muscles are contracted. Staring at an object far away feels relaxing and comfortable because the ciliary muscles are relaxed.
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