POWER OF ACCOMMODATION–
The eye lens is composed of fibrous, jelly-like material. Its curvature can be modified to some extent by the ciliary muscles.
The change in the curvature of the eye lens change focal length. When the muscles are relaxed, the lens can thus change its length.
When you are looking at objects closer to the eye, the ciliary muscles contract. This increases the curvature of the eye lens. The eye lens then becomes thicker.
Consequently, the focal length of the eye lens decreases. This enables us to see nearby objects clearly.
The ability of the eye lens to adjust its focal length is called accommodation. However, the focal length if the eye lens cannot be decreased below a certain minimum limit.
Try to read a printed page by holding it very close. You may see the image being blurred or feel strain in the eye. To see an object comfortably and distinctly, you must hold it about 25 from the eyes.
The minimum distance, at which objects can be seen most distinctly without strain, is called the least distance of distinct vision. It is also called the near point of the eye, 25cm. The furthest point up to which the eye can see objects clearly is called the far point of the eye. It is infinity for the normal eye. You may note here a normal eye can see objects clearly that are between 25 cm and infinity.
Sometimes, the crystalline men's of people at old age become milky and cloudy. This condition is called a cataract. This causes partial or complete loss of vision. It is possible to restore vision through a cataract surgery.
Sometimes, the eye may gradually lose it's the power of accommodation. In such conditions, the person becomes blurred due to refractive defects of the eye.
EYE DEFECTS OR DEFECTS OF EYE—
There are mainly three common refractive defects of vision. These are-
i) Myopia or near-sightedness
ii) Hypermetropia or far-sightedness, and
iii) Presbyopia
These defects can be corrected by the use of suitable spherical lenses.
Eye defects are the problems of the eye which occur in different ages.
The defects due to which a person cannot see the object distinctly and comfortably are called defects of vision.
HOW THIS DEFECT
OCCUR?
Whenever light passes through
eyelets it refracts, it image forms on
retina this causes nothing but sometimes light refract the retina and sometimes it images form in front
of the retina instead of on the retina due to this mesh, some defects may occur.
MYOPIA :
* Myopia is also known as near-sightedness.
* It is a defect in which a person not able to see the distant objects. In this case, image forms before the retina and not on the retina.
* A person with this defect has the far point nearer than infinity.
* Such a person may see clearly up to a distance of a few meters.
* It occurs when the image in front of the retina.
* It is corrected by using concave lens. A concave lens of suitable power will bring back the image on the retina.
*The main causes are.....
A person with this defect has a fair point nearer than infinity. This defect arises due to the decrease in focal length of the lens because of
##. Excessive curvature of the eye lens.
##. Elongation of the eyeball.
HYPERMETROPIA:
* It is a defect in which a person not able to see nearby objects clearly. A person with this defect has the near point farther away from normal near point 25 cm. In this case, the image is formed beyond the retina.
* Such a person has to keep reading material much beyond 25 cm from the eye for a comfortable table.
* This is because the light rays from a close object are focussed at a point behind the retina.
* It is also known as far-sightedness.
* It is corrected by using the convex lens of suitable power. This will bring the image back on the retina.
* Eyeglasses with converging lenses provide the additional focusing power required for forming the image on the retina.
* This defect arises due to following reasons...
##. The focal length of the eye lens becomes large.
##. Eyeball becomes too short so that the image is formed behind the retina.
PRESBYOPIA:
* The power of the accommodation of the eye usually decreases with ageing.
* In this a person suffers from both myopia and hypermetropia. For most of the people, the near point gradually recedes away with age.
* They find it difficult to see nearby objects comfortably and distinctly without corrective eyeglasses.
* Sometimes, a person may suffer from both myopia and hypermetropia.
* It can be corrected by using bifocal or varifocal lenses which consist of both convex and concave lenses. The upper portion consists of a concave lens(for myopia) and lower portion consists of the convex lens(for hypermetropia).
* In these days, it is possible to correct the refractive defects with contact lenses or through surgical interventions.
This defect arises due to the following reasons...
##. The weakness of ciliary muscles.
##. Hardening or loss of elasticity of eye lens.
CATARACT—
It is a condition in which the crystalline lens of the eye becomes milky and cloudy due to the growth of membrane over it. It generally occurs among people of old age. This causes partial or complete loss of vision. It is possible to restore vision through cataract surgery.
Why do we have two eyes for vision and not just one?
There are several advantages of our two eyes instead of one. It gives a wider field of view. A human being has a horizontal field of view of about 150°with one eye and about 180° with two eyes. The ability to detect faint objects is, of course, enhanced with two detectors instead of one.
Some animals, usually prey animals, have their two eyes positioned on opposite sides of their heads to give the widest possible field of view, But our two eyes are positioned on the front of view in favour of what is called stereopsis.
Shut one eye and the world looks flat– two-dimensional. Keep both eyes open and the world takes on the third dimension of depth.
Because our eyes are separated by a few centimetres, each eye sees a slightly different image. Our brain combines the two image into one, using the extra information to tell us how close or far away things are.
I hope this artical is proved to be benefitted.
The eye lens is composed of fibrous, jelly-like material. Its curvature can be modified to some extent by the ciliary muscles.
The change in the curvature of the eye lens change focal length. When the muscles are relaxed, the lens can thus change its length.
When you are looking at objects closer to the eye, the ciliary muscles contract. This increases the curvature of the eye lens. The eye lens then becomes thicker.
Consequently, the focal length of the eye lens decreases. This enables us to see nearby objects clearly.
The ability of the eye lens to adjust its focal length is called accommodation. However, the focal length if the eye lens cannot be decreased below a certain minimum limit.
Try to read a printed page by holding it very close. You may see the image being blurred or feel strain in the eye. To see an object comfortably and distinctly, you must hold it about 25 from the eyes.
The minimum distance, at which objects can be seen most distinctly without strain, is called the least distance of distinct vision. It is also called the near point of the eye, 25cm. The furthest point up to which the eye can see objects clearly is called the far point of the eye. It is infinity for the normal eye. You may note here a normal eye can see objects clearly that are between 25 cm and infinity.
Sometimes, the crystalline men's of people at old age become milky and cloudy. This condition is called a cataract. This causes partial or complete loss of vision. It is possible to restore vision through a cataract surgery.
Sometimes, the eye may gradually lose it's the power of accommodation. In such conditions, the person becomes blurred due to refractive defects of the eye.
EYE DEFECTS OR DEFECTS OF EYE—
There are mainly three common refractive defects of vision. These are-
i) Myopia or near-sightedness
ii) Hypermetropia or far-sightedness, and
iii) Presbyopia
These defects can be corrected by the use of suitable spherical lenses.
Eye defects are the problems of the eye which occur in different ages.
The defects due to which a person cannot see the object distinctly and comfortably are called defects of vision.
HOW THIS DEFECT
OCCUR?
Whenever light passes through
eyelets it refracts, it image forms on
retina this causes nothing but sometimes light refract the retina and sometimes it images form in front
of the retina instead of on the retina due to this mesh, some defects may occur.
MYOPIA :
* Myopia is also known as near-sightedness.
* It is a defect in which a person not able to see the distant objects. In this case, image forms before the retina and not on the retina.
* A person with this defect has the far point nearer than infinity.
* Such a person may see clearly up to a distance of a few meters.
* It occurs when the image in front of the retina.
* It is corrected by using concave lens. A concave lens of suitable power will bring back the image on the retina.
*The main causes are.....
A person with this defect has a fair point nearer than infinity. This defect arises due to the decrease in focal length of the lens because of
##. Excessive curvature of the eye lens.
##. Elongation of the eyeball.
HYPERMETROPIA:
* It is a defect in which a person not able to see nearby objects clearly. A person with this defect has the near point farther away from normal near point 25 cm. In this case, the image is formed beyond the retina.
* Such a person has to keep reading material much beyond 25 cm from the eye for a comfortable table.
* This is because the light rays from a close object are focussed at a point behind the retina.
* It is also known as far-sightedness.
* It is corrected by using the convex lens of suitable power. This will bring the image back on the retina.
* Eyeglasses with converging lenses provide the additional focusing power required for forming the image on the retina.
* This defect arises due to following reasons...
##. The focal length of the eye lens becomes large.
##. Eyeball becomes too short so that the image is formed behind the retina.
PRESBYOPIA:
* The power of the accommodation of the eye usually decreases with ageing.
* In this a person suffers from both myopia and hypermetropia. For most of the people, the near point gradually recedes away with age.
* They find it difficult to see nearby objects comfortably and distinctly without corrective eyeglasses.
* Sometimes, a person may suffer from both myopia and hypermetropia.
* It can be corrected by using bifocal or varifocal lenses which consist of both convex and concave lenses. The upper portion consists of a concave lens(for myopia) and lower portion consists of the convex lens(for hypermetropia).
* In these days, it is possible to correct the refractive defects with contact lenses or through surgical interventions.
This defect arises due to the following reasons...
##. The weakness of ciliary muscles.
##. Hardening or loss of elasticity of eye lens.
CATARACT—
It is a condition in which the crystalline lens of the eye becomes milky and cloudy due to the growth of membrane over it. It generally occurs among people of old age. This causes partial or complete loss of vision. It is possible to restore vision through cataract surgery.
Why do we have two eyes for vision and not just one?
There are several advantages of our two eyes instead of one. It gives a wider field of view. A human being has a horizontal field of view of about 150°with one eye and about 180° with two eyes. The ability to detect faint objects is, of course, enhanced with two detectors instead of one.
Some animals, usually prey animals, have their two eyes positioned on opposite sides of their heads to give the widest possible field of view, But our two eyes are positioned on the front of view in favour of what is called stereopsis.
Shut one eye and the world looks flat– two-dimensional. Keep both eyes open and the world takes on the third dimension of depth.
Because our eyes are separated by a few centimetres, each eye sees a slightly different image. Our brain combines the two image into one, using the extra information to tell us how close or far away things are.
I hope this artical is proved to be benefitted.
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