Nearsightedness (myopia)
myopia is common name for impaired vision in which a person sees near objects clearly while distant objects appear blurred. In such a defective eye, the image of a distant object is formed in front of the retina and not at the retina itself. Consequently, a nearsighted person cannot focus clearly on an object farther away than the far point for the defective eye.


Causes:
This defect arises because the power of the eye is too great due to the decrease in focal length of the crystalline lens. This may arise due to either

1) excessive curvature of the cornea, or
2) elongation of the eyeball.

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Nearsighted, myopic eye
Nearsighted, myopic eye




Correction :-
This defect can be corrected by using a concave (diverging) lens. A concave lens of appropriate power or focal length is able to bring the image of the object back on the retina itself.

Farsightedness (hyperopia)
Farsightedness, also called hypermetropia, common name for a defect in vision in which a person sees near objects with blurred vision, while distant objects appear in sharp focus. In this case, the image is formed behind the retina.


Causes:
This defect arises because either
(i) the focal length of the eyelens is too great, or
(ii) the eyeball becomes too short, so that light rays from the nearby object, say at point N, cannot be brought to focus on the retina to give a distinct image.


Farsighted, hyperopic eye
Farsighted, hyperopic eye

Hyperopic (Farsighted) Eye

The Image is Formed at a Theoretical Point Behind the Eye

Correction:-
This defect can be corrected by using a convex (converging) lens of appropriate focal length. When the object is at N’, the eye exerts its maximum power of accommodation. Eyeglasses with converging lenses supply the additional focussing power required for forming the image on the retina.
Astigmatism
Astigmatism, a defect in the outer curvature on the surface of the eye that causes distorted vision.In astigmatism, a person cannot simultaneously focus on both horizontal and vertical lines.
Causes:
This defect is usually due to the cornea that is not perfectly spherical. Consequently, it has different curvatures in different directions in vertical and horizontal planes. This results in objects in one direction being well-focussed, while those in a perpendicular direction not wellfocussed.



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Astigmatism
Astigmatism

Astigmatism: The cornea is steeper in one meridian and flatter in the opposite meridian 180 degrees away, resulting in two planes of focus


Correction:-
This defect can be corrected by using eyeglasses with cylindrical lenses oriented to compensate for the irregularities in the cornea.