There are four types of defects of the Eye: Myopia, Hypermetropia, Presbyopia and Astigmatism. Below are given the nature of the defect, its causes and corrective measures:-
Nearsightedness, also called myopia is a 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
(i) an excessive curvature of the cornea, or
(ii) elongation of the eyeball.
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.
Method for calculating the power of the corrective lens: –
For calculating the required power of a corrective lens, we first find the power of the eye at its far point. Then, we select a corrective lens of appropriate power to move the far point to infinity. We then use the thin lens formula, written in terms of power P of the lens as the image distance v of the eye can be taken as 0.02 m approximately.
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 eye lens 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.
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 focusing power required for forming the image on the retina.
Presbyopia, progressive form of farsightedness that affects most people by their early 60s. The power of accommodation of the eye decreases with ageing. Most people find that the near point gradually recedes.
Cause and cure:
It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the crystalline lens. Simple reading eyeglasses with convex lenses correct most cases of presbyopia. Sometimes, a person may suffer from both myopia and hypermetropia. Such people often require bi-focal lenses. In the bi-focal lens, the upper portion of the bifocal lens is a concave lens, used for distant vision. The lower part of the bifocal lens is a convex lens, used for reading purposes.
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-focused, while those in a perpendicular direction not well focused.
This defect can be corrected by using eyeglasses with cylindrical lenses oriented to compensate for the irregularities in the cornea.