A person is unable to read a book placed closer than 1 meter from his eyes. Identify the defect of vision in his eyes. Draw the ray diagrams to show the defect of vision and its correction.
A person is unable to read a book placed closer than 1 meter from his eyes. Identify the defect of vision in his eyes. Draw the ray diagrams to show the defect of vision and its correction.
The person is suffering from hypermetropia (far-sightedness), since his near point has receded from the normal 25 cm to 1 m (100 cm) — he cannot focus on objects closer than 1 m. This defect is corrected using a convex (converging) lens of power +3 D (focal length ≈ 33.3 cm), placed in front of the eye.
Marking Scheme
- 11 mark: Correct identification of the defect as hypermetropia, with reasoning (near point receded from 25 cm to 1 m).
- 21 mark: Correct, clearly labelled ray diagrams for (i) the uncorrected hypermetropic eye showing the image forming behind the retina, and (ii) the corrected eye showing a convex lens forming the image on the retina.
- 31 mark: Correct method/lens type stated (convex lens) — full numerical calculation of power (+3 D) may be awarded as bonus/part of complete answer depending on marking scheme used by the school.
Hint
Compare the person's near point (1 m) with the normal near point (25 cm). Which lens type increases the eye's converging power? Use the lens formula with the object at the normal near point and the required image at the person's actual near point.
Quick Oral Answer
The near point has shifted from 25 cm to 1 m, so this is hypermetropia. A convex lens of power +3 D is needed: it converges the diverging rays from an object at 25 cm just enough so that they appear to come from the person's actual near point of 1 m, letting the eye's own lens focus them on the retina.
Analysis & Explanation
A person with normal vision can read comfortably down to the near point of 25 cm (least distance of distinct vision). This person cannot read anything closer than 1 m — his near point has receded far beyond 25 cm. As per the NCERT text: "A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly... because the light rays from a closeby object are focussed at a point behind the retina." This matches the given condition exactly, so the defect is hypermetropia (far-sightedness). It is corrected using a convex lens of appropriate power, which converges the incoming rays before they enter the eye so that the eye's own lens can focus them on the retina.
Common Mistakes
- 1Confusing hypermetropia with myopia because the person 'cannot see something clearly' — myopia is inability to see distant objects, not near ones; here the defect is inability to see NEAR objects, which is hypermetropia.
- 2Prescribing a concave lens (used for myopia) instead of the required convex lens.
- 3Sign errors in the lens formula — forgetting that both u and v are negative for a virtual image formed on the same side as the object (Cartesian sign convention), leading to a wrong value or wrong sign of power.
- 4Forgetting to convert focal length from cm to metres before computing power, giving an answer off by a factor of 100.
Interesting Facts
Hypermetropia can arise either because the focal length of the eye lens is too long or because the eyeball has become too small — in both cases, the image of a close object forms behind the retina instead of on it.
People suffering from both myopia and hypermetropia together often need bi-focal lenses — the upper concave part corrects distance vision, and the lower convex part corrects near vision.
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Frequently Asked Questions
How many marks does this question carry in CBSE Class 10 Science 2026?
This question carries 3 marks in the CBSE Class 10 Science 2026 examination.
Which chapter does this question come from in Science?
This question is from the chapter "The Human Eye and the Colourful World" in the CBSE Class 10 Science syllabus.
What topic does this question cover in Science?
This question covers the topic "Defects of Vision and their Correction — Hypermetropia" from CBSE Class 10 Science.
What type of question is this in the CBSE Class 10 Science 2026 paper?
This is a Short Answer question from Section C in the CBSE Class 10 Science 2026 paper.