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The power of a lens is −0.25… (Class 10 Science 2025, Q31)

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ScienceCBSE Class 10 · Board exam 2025, set 31/5/1 · 3 marks · Short answer

CBSE Class 10 Science 2025: question 31

Set in the CBSE Class 10 board exam of 2025, paper 31/5/1, section C, question 31, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q31 · 3 marks · Short answerLight – Reflection and Refraction

The power of a lens is −0.25 D. Based on this information, find out

  1. (a)

    The type of lens and its focal length.

    0 marks
  2. (b)

    The eye defect for which it may be used as a corrective lens.

    0 marks
  3. (c)

    The nature and size of the image formed by this lens when an object is placed between F and 2F from the optical centre of this lens.

    0 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/5/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

The sign of the power tells you the type of lens.

Hint 2

Use with f in metres; which eye defect needs a diverging lens?

Hint 3

A diverging lens gives the same kind of image for every object position.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. (a) The power is negative, so the lens is a concave (diverging) lens.

    ½ mark

  2. (a) (i.e. cm).

    ½ mark

  3. (b) A concave lens is used to correct myopia (near-sightedness).

    1 mark

  4. (c) Nature: the image is virtual and erect. A concave lens always forms such an image, wherever the object is placed.

    ½ mark

  5. (c) Size: the image is diminished (smaller than the object) and forms between the optical centre and the focus on the same side as the object.

    ½ mark

Answer: (a) Concave lens, f = −4 m; (b) myopia; (c) virtual, erect and diminished

Where the marks are

  • Identifies concave lens (½ mark)
  • f = 1/P = −4 m (½ mark)
  • Myopia (1 mark)
  • Virtual and erect (½ mark)
  • Diminished (½ mark)

Revise the chapter

Practise this chapterBoard-style questions from Light – Reflection and Refraction, one at a time, marked against the answer key and marking points.Back to the full paper2025 board paper, set 31/5/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.