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.