Power of Accommodation

The ability of lens to adjust the focal length,in order to form clear image on retina is called as power of accommodation.

Distance of Distinct Vision

The focal length of lens could not be decreased below a certain value. The minimum distance from the normal eye at which an object can be seen clearly is called distance of distinct vision. For normal eye it is 25cm. It is denoted by D.

Vision Defects – Myopia or Nearsightedness

Person suffering from myopia can see nearby objects clearly however, fails to see distant object clearly. This happens because of weakening of ciliary muscle. Ciliary muscle do not relax sufficiently, this increases the curvature of eye lens thereby decreasing the focal length. Myopia is also caused when length of eyeball increases and image is formed before retina. In order to correct this defect, there is need to diverge the light rays falling on lens hence, a concave lens of suitable power used in spectacles. Since, the focal length of the concave lens is negative. Thus power of spectacles for myopic eye is negative.

Vision defects – Hypermetropia or Long sightedness

Person suffering from hypermetropia can see long distance object clearly but cannot see nearby objects clearly. A hypermetropic eye forms image behind the retina. It is caused due to weakening of ciliary muscle. Ciliary muscles in the eye donot contract sufficiently , thus curvature of lens decrease thereby increasing the focal length. However, hypermetropia is also caused if there is decrease in length of eyeball then the distance between lens and retina decreases. Hence, converging power of lens decreases , thus forming image behind retina. To correct hypermetropia, there is need to converge the light rays falling on the lens hence a convex lens of suitable power used in spectacles. As, focal length of convex lens is positive thus power of spectacles for hypermetropia eye is positive.

Vision defects – Presbyopia or Old age hypermetropia

With increase in age power of accommodation of eye decreases. Thus it becomes difficult to see nearby object. This defect is caused due to weakening of ciliary muscles. Thus ciliary muscle do not contract sufficiently and hence curvature of lens decreases thereby increasing the focal length. Sometime old people suffers from myopia and hypermetropia. In order to correct this defect bifocal lens is used. In bifocal lens upper part is concave lens and lower part is convex to correct myopia and hypermetropia respectively.

Vision defects – Astigmatism

Normal eye can see all lines with equal intensity. For astigmatic eye there is variation in intensity of different lines. This defect is because cornea of eye is not spherical. With help of cylindrical lens of specified radius of curvature and appropriately directed axis, astigmatism can be corrected.

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Keywords: Lens, Eye defects, Eye problems, Vision problems, Astigmatism, Presbyopia, Hypermetropia, Myopia