Which one among the following statements related to human eye is correct?

Correct — C, For a person suffering from long-sight defect, the image of a near object is focused behind the retina. Long sight, or hypermetropia, arises when the eyeball is too short or the lens too weak, so converging power is insufficient and rays from a nearby object are still converging when they reach the retina — the focus falls behind it, and the near point recedes beyond the normal 25 centimetres. A converging, that is convex, lens supplies the missing power and brings the image forward onto the retina. Each of the other three statements reverses either the position of the image or the type of correcting lens.

• (a)For a person suffering from short-sight defect, the image of a distant object is focused behind the retina. — The opposite. In myopia the eye converges too strongly or the eyeball is too long, so a distant object focuses in front of the retina, not behind it.
• (b)Short-sight defect can be corrected by a converging spectacle lens. — Wrong lens. Myopia is over-convergence, so it is corrected by a diverging, that is concave, lens of suitable negative power, which pushes the image back onto the retina.
• (d)Long-sight defect can be corrected by a diverging spectacle lens. — Also the wrong lens. Hypermetropia is under-convergence and needs a converging convex lens; a diverging lens would make the defect worse.

A normal eye focuses parallel rays from a distant object exactly on the retina and, by accommodation, can also focus objects down to about 25 centimetres, the near point. Myopia, or short sight, means distant objects focus in front of the retina, usually because the eyeball is elongated; the far point comes closer than infinity and a concave lens corrects it. Hypermetropia, or long sight, means near objects focus behind the retina, usually because the eyeball is short; the near point moves further away and a convex lens corrects it. Presbyopia is the age-related stiffening of the lens that weakens accommodation, and it often needs bifocals.

All four statements are built from two binary choices — in front of or behind the retina, and converging or diverging lens — so the item can be worked with a single principle rather than four separate memories. Short sight is too much converging power, so the image lands early, in front, and the remedy must take power away, which is a concave lens. Long sight is too little, so the image lands late, behind, and the remedy must add power, which is a convex lens. Applying that rule to each statement leaves only one standing.

• Myopia: distant objects focus in front of the retina; corrected by a concave (diverging) lens.
• Hypermetropia: near objects focus behind the retina; corrected by a convex (converging) lens.
• The near point of a normal eye is about 25 centimetres and the far point is at infinity.
• Presbyopia results from the loss of accommodation with age and often requires bifocal lenses.
• The power of a corrective lens is given in dioptres, negative for myopia and positive for hypermetropia.

• Swapping the two lens types — this is the single most common error in the topic.
• Assuming long sight means good distance vision and nothing else; the defining fault is the receded near point.
• Confusing presbyopia with hypermetropia; one is age-related loss of accommodation, the other a refractive error of the eyeball.

A four-statement item built from two binary variables, so it can be solved by rule rather than by recall of four separate facts.

• practice — not a real PYQ

  A person who cannot see distant objects clearly is suffering from a defect corrected by

  • (a)a convex lens
  • (b)a concave lens
  • (c)a cylindrical lens
  • (d)a bifocal lens

  Answer(b) a concave lens — the defect is myopia, an excess of converging power, and a diverging lens removes the excess.

• practice — not a real PYQ

  The near point of a person with hypermetropia is

  • (a)nearer than 25 cm
  • (b)exactly 25 cm
  • (c)farther than 25 cm
  • (d)at infinity

  Answer(c) farther than 25 cm — the eye cannot converge enough for close work, so the closest point of distinct vision recedes.