Question

Name the type of lens which is used to correct?
(i) Myopia and (ii) Hypermetropia.

Answer 1

Hint: Myopia and Hypermetropia are two common defects of vision also called refractive defects which are caused by incorrect refraction of light by the eye lens. In myopia (or shortsightedness) a person can see nearby objects clearly but cannot see the distant object. The converging power of eye-lens in the myopic eye increases this will result in the formation of a distant object in front of the retina. In the case of Hypermetropia (or long-sightedness) a person cannot see nearby objects clearly but can see the distant object. The converging power of eye-lens in this defect increases this will result in the formation of the distant object behind the retina.

Complete step by step solution:
(i) Myopia – The defect of vision in human eyes in which a person can see nearby objects clearly but cannot see the distant object. Hence, it is also called Short-sightedness. In myopia the far point of the eye is less than infinity.
Cause of defect:
(a) The converging power of the eye get increases or
(b) Size of the eyeball increases.
Due to these reasons, the image of distant objects is formed in front of the retina.

The myopia vision defect can be corrected by using a concave lens of suitable power in the front of the myopic eye.
(ii) Hypermetropia - The defect of vision in human eyes in which a person cannot see nearby objects clearly but can see the distant objects. Hence, it is also called long-sightedness. In hypermetropia near the point of the eye that is more than 25 centimeters away it is opposite to myopia.
Cause of defect:
(a) The low converging power of the eye get increases or
(b) Decreased size of the eyeball.
Due to these reasons, the image of a distant object is formed behind the retina.

The Hypermetropia vision defect can be corrected by using a convex lens of suitable power in the front of the myopic eye.

$\therefore$ Myopia can be treated by the concave lens and Hypermetropia can be treated using the convex lens.

Note:
Students often confuse the meaning of terms myopia and Hypermetropia. Due to the Synonyms of long-sightedness and short-sightedness, it is a common mistake to understand that short-sightedness means a defect of vision in which something is wrong in the near point of the eye and the vice versa for the Hypermetropia. To tackle such misconceptions one should remember these defects in terms of the power of the eye lens. When the power of the eye lens is increased the vision defect is known as myopia while when it decreases we call it Hypermetropia.