Question

What are the defects of vision? Write down the names of the lenses used for the correction of vision defect of three persons P, Q, and R if —
(i) \( P \) can see only near objects clearly but cannot see far objects.
(ii) \( Q \) can see only far objects clearly but cannot see near objects.
(iii) \( R \) can neither see far objects nor near objects clearly.

Hint:

Remember: - Myopia → Concave lens (short-sightedness) - Hypermetropia → Convex lens (long-sightedness) - Presbyopia → Bifocal lens (both near & far defects)

Answer 1

Step 1: Understanding the defects of vision.
There are three common defects of vision — Myopia, Hypermetropia, and Presbyopia. These occur due to improper focusing of light on the retina by the eye lens.
(i) Myopia (Short-sightedness):
A person with myopia can see nearby objects clearly but not distant objects. This defect occurs when the image of a distant object is formed in front of the retina because the eyeball is elongated or the eye lens is too curved.
Correction: A concave lens (diverging lens) is used to diverge the incoming light rays so that the image forms correctly on the retina.
(ii) Hypermetropia (Long-sightedness):
A person with hypermetropia can see distant objects clearly but not near objects. This happens when the image of a nearby object is formed behind the retina due to a short eyeball or less converging power of the eye lens.
Correction: A convex lens (converging lens) is used to converge the incoming light rays before they enter the eye.
(iii) Presbyopia:
This defect occurs in old age due to loss of flexibility of the eye lens. The person cannot see both near and distant objects clearly.
Correction: Bifocal lenses are used — the upper part is concave (for distant vision) and the lower part is convex (for near vision).
Step 2: Conclusion.
\[ \text{P — Myopia → Concave lens}
\text{Q — Hypermetropia → Convex lens}
\text{R — Presbyopia → Bifocal lens} \]