SENSE OF SIGHT
Some words in this article maybe new to some readers, so it’s advice to use this picture above as a guideline.
The sense of sight or vision enables us to see our external environment. It provides the richest and most detailed source of sensory information. It tells us the things that are around us give their location; and provide three-dimensional images of them(in color or motion).
Each eye is a spherical organ known as eyeball. It is located in a bony sockets in the skull. Tear glands under the upper eyelid produce a saline tear gland(made up of sodium chloride and hydrogen carbonate). We blink automatically every two to ten seconds. This action spreads tear fluid over the eye surfaces cleaning and keeping moist. Tear fluid contains a lysozyme which attack germs.
How We See
When we look at an object light rays from each point if the object enter our eyes. As they pass through the cornea, aqueous humor, lens and vitreous humour. They are bent and brought to a focus o the retina to form an image, then pattern of electrical impulse is sent to the brain which interprets it, drawing on past experience. The brain tells us that the object is upright and gives us its real size and distance from the eye. We therefore see the objects as it actually is, and not as the image formed on the retina.
Common Eye Defect And Their Correction
These are the common eye defect:
They are mainly caused by excessive eye strain and old ages.
Short-Sightedness This condition is also known as near-sightedness or myopia. People who are short-sighted can see nearby object clearly but distance ones appear blurred. This is usually due to their eyeball being longer than normal from back to front. As a result, light ray from distance object are brought to focus in front of the retina, making the image formed blurred.
Shortsightedness can be corrected by using spectacles or contact lenses with suitable concave surface. These will diverge the rays entering the eyes to the correct extent and bring them to a focus on the retina.
NOTE: Contact lenses work exactly like spectacles expect that they are placed directly in contact with the cornea. They are small and thin, and made of clear soft or hard plastic.
Long-Sightedness people who are long-sighted can see distant object clearly but nearby one’s apear blurred. This maybe due to hypermetropia or presbyopia.
Long-sightedness can be corrected by using spectacles or contact lenses with suitable convex surfaces. These will converge the rays entering the eyes to the correct extent and bring them to a focus on the retina.
Astigmatism In some people, the surface of the cornea is uneven. It is not curved smoothly, light rays entering such an eye is bend unevenly producing an image that is stretched or distorted.
Astigmatism is a very common eye defect and is usually present in all eyes to a small extent. In certain cases of astigmatism, vertical figures can be seen clearly but horizontal figures appear blurred. Astigmatism maybe corrected by using lenses with Compensating uneven surface.
Colour-Vision There are there basic types of cone cells, one type is stimulated by red light, the second by green light and the third by blue-violet light. All other colour we see are combinations obtained by stimulate these cone cells together.
For example, we see a purple coloured object the brain receives impulses simultaneously from red and blue-violet cone cells. In colour-blindness, one or more types of cones are absent or defective. A common colour blind condition is red-green col our a blindness where a person sees a red or a green object as grey.
Note: colour-blindness is an inherited condition.
Other eye defects maybe due to
*vitamin A deficiency
*Infection caused by bacteria and virus and
*Poor hygiene habits
Cataract This condition occurs mainly in old people. The lens becomes cloudy, sufficient light connot pass through it so that the affected person cannot see clearly.it can be corrected by removing the affected lens and either
*replacing it with suitable lens
*providing spectacles with suitable lens
Night-blindness people with night- blindness cannot see clearly in dim light. It may be due o a deficiency of vitamin A, the substance used to make rhodopsin.
Xerophthalmia This is an eye disease cause by vitamin A deficiency. Foods that contain vitamin A are
- Fish-liver oil, palm oil, milk, egg yolks, green and yellow vegetables and fruits
*bacteria (a very contagious infection)
Charts Used To Test Defective Vision
Chart for testing colour blindness
A person with normal color vision should be able to see this numbers from any close distance clearly.
Chart for testing long-sightedness
A person with normal vision standing at a distance of 6m from the chart should be able to read the letter in sixth row without any problem
Chart used for testing astigmatism
A person with astigmatism will not be able to see one or more of the line in the chart clearly
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