The Eye (OCR GCSE Biology A (Gateway))
Revision Note
The Human Eye
The eye is a sense organ containing receptors sensitive to light intensity and colour
Receptors are groups of specialised cells that can generate an electrical impulse in a sensory neurone
The eye contains two types of receptor cell: rod cells which are sensitive to light intensity and cone cells which are sensitive to different wavelengths of visible light (colour)
The structure of the eye
The purpose the eye is to receive light and focus it onto the retina at the back of the eye
The retina is where the rod and cone cells are located
The eye is a sensory organ made up of several different tissue types
Eye structure & function table
The pupil reflex
This is a reflex action carried out to protect the retina from damage in bright light and protect us from not seeing objects in dim light
In dim light the pupil dilates (widens) in order to allow as much light into the eye as possible
In bright light the pupil constricts (narrows) in order to prevent too much light entering the eye and damaging the retina
The pupil reflex in bright light
The pupil reflex in dim light
Focussing 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
Diagram showing the eye when an object is close up
Diagram showing the eye when an object is far away
To focus on a close up object:
Ciliary muscles contract
Suspensory ligaments slacken
This allows lens to become fatter
Light is refracted more
To focus on a distant object:
Ciliary muscles relax
Suspensory ligaments are pulled tight
The lens is pulled thinner
Light is refracted less
Table summarising the changes in the eye when focusing on close or distant objects
Common Eye Defects
Short-sightedness
Short-sightedness is also called myopia
It happens when the lens is more curved than normal or the eyeball is too long which means the light is refracted too much and so the Focal Point falls in front of the retina (rather than on the retina)
This means that distant objects appear blurry
Short-sightedness occurs when light is refracted too much
Treatment of short-sightedness
Short-sightedness can be corrected using contact lenses or glasses with a concave lens
Short-sightedness is treated with a concave lens
Long-sightedness
Individuals who suffer from long-sightedness can see distant objects clearly
It happens when the lens is less curved than normal or the eyeball is too short which means the light is not refracted enough and so the focal point falls behind the retina (rather than on the retina)
This means that close objects appear blurry
Long-sightedness occurs when light is not refracted enough
Treatment of long-sightedness
Long-sightedness can be corrected using contact lenses or glasses with a convex lens
Long-sightedness is treated using a convex lens
Colour blindness
People who suffer from colour blindness cannot distinguish between certain colours and in rare cases, cannot see colours at all
This happens because the cones in the retina do not work properly or are absent
It is a genetically inherited condition but can also develop over time
There are several different types of colour blindness, the most common is red-green colour blindness
Treatment of colour-blindness
There is currently no cure for colour blindness (as the cone cells cannot be replaced) so most sufferers learn to live with the condition
Some sufferers choose to use tinted lenses to emphasize some colours more
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