The ability of an eye to adjust its focal length by itself is known as power of accomodation of the eye.
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The process by which the ciliary muscles change the focal length of an eye lens to focus distant or near objects clearly on the retina is called the accommodation of the eye.
Power of Accommodation
The ability of the eye to focus objects lying at different distances is called the power of accommodation of the eye.
How Does an Eye Focus Objects at Varying Distances?
To focus on distant objects the ciliary muscles relax making the eye lens thin.
As a result the focal length of the eye lens increases and we see the distant objects.
But to focus on nearby objects the ciliary muscles contract making the eye lens thick. As a result the focal length of the eye lens decreases and we see the nearby objects.
In short it is the adjustment of the focal length of the eye lens which enables us to focus on objects situated at different distances.
Near point or Least Distance of Distinct Vision
Near point or least distance of distinct vision is the point nearest to the eye at which an object is visible distinctly.
For a normal eye the least distance of distinct vision is about 25 centimetres.
However, it varies with age of the person. For example, for infants it is only 5 to 8 cm.
Far Point
Far point of the eye is the maximum distance up to which the normal eye can see things clearly. It is infinity for a normal eye.
Range of Vision
The distance between the near point and the far point is called the range of vision.
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The process by which the ciliary muscles change the focal length of an eye lens to focus distant or near objects clearly on the retina is called the accommodation of the eye. The ability of the eye to focus objects lying at different distances is called the power of accommodation of the eye.
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The eye lens is composed of a 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 can thus change its focal length. When the muscles are relaxed, the lens becomes thin. Thus, its focal length increases. This enables us to see distant objects clearly. 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 of the eye lens cannot be decreased below a certain minimum limit. Try to read a printed page by holding it very close to your eyes. You may see the image being blurred or feel strain in the eye. To see an object comfortably and distinctly, you must hold it at about 25 cm 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. For a young adult with normal vision, the near point is about 25 cm. The farthest point upto which the eye can see objects clearly is called the far point of the eye. It is infinity for a normal eye. You may note here a normal eye can see objects clearly that are between 25 cm and infinity.
Sometimes, the crystalline lens of people at old age becomes milky and cloudy. This condition is called cataract. This causes partial or complete loss of vision. It is possible to restore vision through a cataract surgery.
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Accommodation of the eye is the ability of the human eye to see sharply all objects which are located at various distances in front of the eye depending on the changes of massiveness of the optical system of the eye. The mechanism of eye accommodation is not the same for all animals. For example fish accommodates through the change of position of the lens, some types of birds accommodate through the increase of curvature of the cornea and protraction of the human eye. As far as humans are concerned accommodation is caused by the increased curvature of anterior area of the eye lens while at the same time its thickness also changes. Accommodation is usually the same on both eyes. For the vision to be sharp it is necessary that the image is located on the retina. The eye is able to see sharply objects at various distances. As it was mentioned before adaptation of the human eye to various distances is called accommodation. The change of shape and thereby also the parameters of the eye lens are caused by ciliary muscles. When observing objects located close to us the radius of curvature of anterior area of the eye lens decreases and at the same time its thickness increases which results in reduction of the focal distance of the optical system of the human eye. (When observing distant objects the ciliary muscles are strained less whereas when observing objects located close to us they are tense and the eye gets tired) The mechanism of eye accommodationTo put it simply we can say that eye accommodation leads to contraction of fibers of the ciliary muscle which leads to relaxation of the lens and thereby the change of the radius of curvature. However, whole process is substantially more complicated. The basic biomechanical and anatomical changes during accommodation include: contraction of the ciliary muscle, the ciliary muscle with the choroid and posterior part of the ciliary suspension move forward by Elastic eye lens and its casing become round and the refrangibility increases because the equatorial diameter of the lens decreases by The process of accommodation of the human eye is therefore influenced by two factors: the ability of the lens to change its shape and the strength of the ciliary muscle. The ability of the lens to change its shape (physical deformation of the lens – compressibility and elasticity) is called physical accommodation. Contraction strength of the ciliary muscle is then called physiological accommodation. Accommodation of the eye is influenced by many other factors including optical (spherical and chromatic aberration, astigmatism) and non-optical ones (the size, the distance). Even mood and will but also illumination and contrast have some influence. |
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read ur text carefully then u will understand that
The ability of an eye to adjust its focal length is known as accomodation
then what is power of accomodation
power of accomodation is the maximum variation in the power of lens i.e. , between infinity and 25 cm
both the terms are different..
understand it..................
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How Does an Eye Focus Objects at Varying Distances?
To focus on distant objects the ciliary muscles relax making the eye lens thin.
As a result the focal length of the eye lens increases and we see the distant objects.
But to focus on nearby objects the ciliary muscles contract making the eye lens thick. As a result the focal length of the eye lens decreases and we see the nearby objects.
In short it is the adjustment of the focal length of the eye lens which enables us to focus on objects situated at different distances.
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