Thursday, 28 February 2013
Defects of vision
Myopia(or) Short sightedness --Human eye can see clearly the objects lying at short distances from it. but not the far off objects
Causes of Myopia
1. Increase in the length of the eye ball as if distance of the retina from the eye has increased.
2. Decrease in focal length of eye lens when the eye is fully relaxed.
Remedy-- To correct a myopic eye, the person has to wear spectacle with a concave lens of suitable focal length.
Hypermetropia (or) Long sightedness-- It is that defect of a human eye in which a person can see clearly the objects lying at large distances from it but the nearby objects cannot be seen clearly.
Causes of Hypermetropia
(1) Decrease in length of eye ball as if distance of retina from the eye lens has decreased
(2) Increase in the focal length of the eye lens when the eye is fully relaxed.
Remedy-- To correct a hypermetropic eye, the person has to wear spectacle with a convex lens of suitable focal length.
Presbyopia-- In this defect old person cannot read and write comfortably.
Cause of Presbiopia--
(1)Gradual weakening of ciliary muscles
(2)Diminishing flexibility of eye lens.
Remedy--An old person has to use spectacles with a convex lens of suitable focal length
When a person suffers from both myopia and hypermetropia his spectacles have bi-focal lenses (i.e) both concave and convex lenses.
AstigmatismThe defect by which the person is notable to differentiate horizontal and vertical position, is called astigmatism.Remedy----It can be rectified by using cylindrical lenses.
Eye lens --- double convex lens
The retina --- contains numerous light sensitive cells which generate electrical signals which are sent to brain via optic nerve
Cornea---The transparent spherical membrane covering the front of the eye.
Iris--The coloured diaphragm between the cornea and lens.
Pupil ---The small hole in the iris.
Eye lens--- It is a transparent lens made of jelly like material.
Ciliary muscles--These muscles hold the lens in position.
Retina--The back surface of the eye.
Blind spot--The point at which the optic nerve leaves the eye. An image formed at this point is not sent to the brain.
Aqueous humour---A clear liquid region between the cornea and the lens.
Vitreous humour--The space between eye lens and retina is is filled with another liquid called Vitreous humour.
Persistence of vision--The image of an object seen persists on the retina for 1/16 second even after the removal of the object. This continuance of sensation of eye for some timed is called persistence of vision.
Colour blindness-- It is said to occur when a person cannot distinguish between colours
Accomodation--The ability of the eye to focus both near and distant objects, by adjusting the focal length, is called the accommodation of the eye.
Far Point of the Eye-- It is the farthest point at which the object can be seen clearly. For a normal eye, the far point lies at infinity.
Near point of the Eye-- It is the closest point at which an object can be seen clearly. For normal eye, the near point lies at 25 cm from the eye(least distance of distinct vision).
Posted 1 hour ago by preeti kamal