A person can see near objects clearly through his eyes but cannot see distant objects clearly. The possible reason(s) for this defect could be : 1. Excessive curvature of the eye lens. 2. The eye-ball has become too small. 3. The focal length of the eye lens is too large. 4. Elongation of the eye-ball. Select the correct answer using the code given below :
- (a)1 only
- (b)2 and 4 only
- (c)1 and 4 only
- (d)1 and 3 only
Correct — C, 1 and 4 only. A person who reads comfortably but cannot make out a distant signboard is short-sighted, and short-sightedness is myopia. The school text that this item is lifted from names exactly two causes for it — excessive curvature of the eye lens, and elongation of the eyeball. Both do the same thing in the end: they make the eye too powerful for its own length, so light from a far-off object is brought to a focus in front of the retina instead of on it, and the far point creeps in from infinity to a few metres. Statements 1 and 4 are those two causes almost word for word. Statements 2 and 3 are the other pair from the very same page, and they belong to the opposite defect — a focal length that is too long and an eyeball that is too small are the two causes of hypermetropia, where the image of a nearby object lands behind the retina. A concave lens corrects the first pair, a convex lens the second.
- (a)1 only — Keeps a genuine cause but drops the other one. Elongation of the eyeball is if anything the commoner reason for myopia, especially the myopia that grows worse through the school years, so statement 4 cannot be left out.
- (b)2 and 4 only — Takes one cause from each defect. A shorter eyeball brings the retina closer to the lens, which leaves the eye under-powered — that is far-sightedness, the exact opposite of what the person in the stem complains about.
- (d)1 and 3 only — Starts correctly with excessive curvature, then adds a focal length that is too long. A longer focal length means a weaker lens, and a weaker eye cannot be the reason distant objects blur — it is the reason near ones do.
The relaxed eye is built to focus parallel rays from a far object exactly on the retina; the ciliary muscle then thickens the lens to pull nearer objects into focus, which is called accommodation. The farthest point the eye can focus is its far point, normally infinity, and the nearest is its near point, normally about 25 cm for a young adult. Myopia shifts the far point inwards; hypermetropia pushes the near point outwards; presbyopia is the age-related loss of accommodation itself, from stiffening of the lens and weakening of the ciliary muscle.
Four statements, two defects, two causes each — the item is a sorting exercise disguised as a recall question, and it is generous to anyone who reasons instead of memorising. Ask of each statement whether it makes the eye too strong or too weak. Sharper curvature raises the power, so it belongs to myopia; a longer eyeball means rays converge before they reach the back of the eye, so it belongs to myopia too. A longer focal length and a shorter eyeball both leave the eye short of power, so both belong to hypermetropia. The trap is statement 3: 'focal length too large' sounds like something going wrong, and a candidate who has not tied focal length to power will grab it. Note also the printed hyphenation — the paper writes 'eye-ball' in statements 2 and 4 where the textbook writes 'eyeball'; nothing turns on it.
- Myopia is near-sightedness: near objects are seen clearly, distant ones are not, and the far point lies nearer than infinity.
- In a myopic eye the image of a distant object forms in front of the retina; a concave lens of suitable power moves it back onto the retina.
- The two textbook causes of myopia are excessive curvature of the eye lens and elongation of the eyeball.
- The two textbook causes of hypermetropia are a focal length of the eye lens that is too long and an eyeball that has become too small; the correction is a convex lens.
- Presbyopia is neither of these — it comes on with age as the ciliary muscles weaken and the lens loses flexibility, and a person with both myopia and presbyopia needs bi-focals, concave above and convex below.
Sharper lens or longer eyeball means too much power, and the answer stops at statements 1 and 4.
- Reading 'focal length too large' as a cause of near-sightedness. Larger focal length means a weaker eye, which blurs near objects, not far ones.
- Assuming an eyeball problem must be one kind of defect. Elongation gives myopia and shortening gives hypermetropia, so the direction matters.
- Confusing presbyopia with hypermetropia because both blur near vision. Presbyopia is a loss of accommodation with age, not a fixed mismatch of power and length.
Either as this kind of statement-sorting item on the causes of a named defect, or as a one-liner asking which lens corrects which defect.
Myopia is a defect in human vision where an image of a
- (a) nearby object is focused beyond the retina.
- (b) nearby object is focused before the retina.
- (c) distant object is focused before the retina.
- (d) distant object is focused beyond the retina.
Answer(c) distant object is focused before the retina.
The same defect approached from the optics rather than the symptom. Once you know the image of a distant object lands in front of the retina, the causes follow — anything that adds power or lengthens the eye will do it, which is exactly statements 1 and 4 here.
- practice — not a real PYQ
A student can read a book comfortably but cannot read what is written on the blackboard. Which lens should be prescribed?
- (a)Convex lens
- (b)Concave lens
- (c)Cylindrical lens
- (d)Bi-focal lens
Answer(b) Concave lens — the student is myopic, so the image of a distant object is falling in front of the retina and a diverging lens is needed to push it back.
- practice — not a real PYQ
Which one of the following is a cause of hypermetropia?
- (a)Elongation of the eyeball
- (b)Excessive curvature of the eye lens
- (c)The eyeball has become too small
- (d)Weakening of the ciliary muscles with age
Answer(c) The eyeball has become too small — that, along with a focal length of the eye lens that is too long, is one of the two standard causes of far-sightedness. Option (d) describes presbyopia.