The uses of lenses to treat the vision defects
the light rays reflect from the body to fall on the eye , The light rays refract when passing through the cornea then the lens ( the convex lens ) , The refracted rays are collected on the retina forming a small , real inverted image .
The optic ( sight ) nerve transmits the image to the brain which re-corrects the image to become erect and in its natural size , The eyeball diameter is the distance between the lens optical center and the retina .
The image must be formed on the retina to see clearly , The normal person can see clearly the near objects ( at a distance not less than 25 cm ) and the far objects ( at a distance up to 6 m ) .
Uses of lenses to treat the vision defects
The vision defects occur because the eye cornea is not always convex or the eye is not always spherical , The most important vision defects are short-sightedness and long-sightedness .
The short-sightedness is a vision defect through which the near objects can be seen clearly but the far objects seem distorted , The images of the far objects do not fall on the retina of the eye but in front of it .
The short-sightedness is due to the increase in the eyeball diameter that causes the retina to be far from the eye lens , and the increase in convexity of the eye lens surface that results a smaller focal length of the eye lens , the parallel rays coming from the far object are collected at a point in front of the retina and disperses after that forming an unclear image .
The short-sightedness is corrected by using a concave lens which diverges the rays coming from the far object , so , the image of the object is formed on the retina , Therefore , short-sighted person needs medical eye glasses with concave lenses .
The long-sightedness is a vision defect through which the far objects only can be seen clearly but the close objects are not seen clearly , The images of close objects do not fall on the eye retina but behind it .
The reasons of long-sightedness
The long-sightedness is due to the decrease of the eyeball diameter that causes the shortness of the radius of the eye sphere , thus the retina is close to the eye lens .
The long-sightedness is due to the decrease of convexity of eye surface that results more focal length , so the rays coming from the near objects are collected in a point behind the eye retina .
The long-sightedness is corrected by using a convex lens which collects the rays coming from the near objects , so , the image of the objects is formed on the retina , Therefore , Long-sighted person needs a medical eye glasses with convex lenses .
The contact lenses
The contact lenses are used instead of the glasses to treat the vision defects , It is very thin lens that made of plastic , and they can stick to the eye cornea by the eye fluid .
Cataract is one of the most dangerous diseases that infects the eye , It is due to the old age , illness , the side effects of drugs , and the genetic readiness .
The result of cataract is the difficulty of vision as a result of the darkness of the eye lens , The treatment of cataract is done through surgery to exchange the eye lens with a plastic lens transplanted permanently in the eye , So , the person can see again and clearly .