Ans :- Ans :- Three commonrefractive defectsof the eye:

Myopia or Near-sightedness

Hypermetropia or Farsightedness

Presbyopia

Myopia

Near-sightedness: A person with Myopia can see nearby objects clearly

A person with myopia cannotsee faraway objects clearly.

The far point for the myopic eye isnearer than infinity

Occurs due toExcessive curvature of the eye lens

Elongation of eyeball

The image of a distance object is formed infront of the retinaand not on the retina

Defected is corrected by usingConcave lensessuch that the lens will bring the image back on to the retina.

Hypermetropia

Far sightedness: A person with Hypermetropia can see faraway objects clearly.

A person withHypermetropiacannot seenearby objects clearly.

The near point of the eye is more than 25cm

This arises mostly during latter stages in life, as a result of the weakening of the ciliary muscles and/or the decreased flexibility of the lens.

Focal length of the eye lens is too long

Eyeball has become too small.

The image of a distance object is formed inbehind the retina and not on the retina

Defected is corrected by usingConvex lensessuch that the lens will bring the image back on to the retina.

Hypermetropia

Far sightedness: A person with Hypermetropia can see faraway objects clearly.

A person withHypermetropiacannot seenearby objects clearly.

The near point of the eye is more than 25cm

This arises mostly during latter stages in life, as a result of the weakening of the ciliary muscles and/or the decreased flexibility of the lens.

Focal length of the eye lens is too long

Eyeball has become too small.

The image of a distance object is formed inbehind the retina and not on the retina

Defected is corrected by usingConvex lensessuch that the lens will bring the image back on to the retina.

Presbyopia

Thepower of accommodation of the eye usually decreases with ageing. The ciliary muscles weaken and thereby the flexibility of the eye lens reduces.

The near point moves away.

Spectacles with convex lenses are recommended