Question

You know that myopia is a common refractive defect of vision. Person with this defect can see only nearby objects. Using concave lens of suitable power this defect is corrected.
(a) Mention other two types of defects like this.
(b) Explain how we can correct it.

Answer 1

(a) and (b):

Hypermetropia:

It is also known as far-sightedness. A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. The near point, for the person, is farther away from the normal near point $\left(25cm\right)$. Such a person has to keep reading material much beyond $25cm$ from the eye for comfortable reading. This is because the light rays from a closeby object are focussed at a point behind the retina. This defect arises either because (i) the focal length of the eye lens is too long, or (ii) the eyeball has become too small. This defect can be corrected by using a convex lens of appropriate power.

Presbyopia:

The power of accommodation of the eye usually decreases with aging. For most people, the near point gradually recedes away. They find it difficult to see nearby objects comfortably and distinctly without corrective eyeglasses. This defect is called Presbyopia. It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens. Sometimes, a person may suffer from both myopia and hypermetropia. Such people often require bifocal lenses.