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Define power of a lens. The… (Class 10 Science 2023, Q31b)

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ScienceCBSE Class 10 · Board exam 2023, set 31/2/1 · 3 marks · Short answer

CBSE Class 10 Science 2023: question 31(b)

Set in the CBSE Class 10 board exam of 2023, paper 31/2/1, section C, question 31(b), for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q31(b) · 3 marks · Short answerLight – Reflection and Refraction

Define power of a lens. The focal length of a lens is −10 cm. Write the nature of the lens and find its power. If an object is placed at a distance of 20 cm from the optical centre of this lens, according to the New Cartesian Sign Convention, what will be the sign of magnification in this case ?

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 31/2/1.

Hints

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

Hint 1

The sign of the focal length tells you whether the lens converges or diverges light.

Hint 2

Convert f into metres before taking its reciprocal.

Hint 3

Is the image formed by this lens erect or inverted? Erect images have positive magnification.

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. Power of a lens: It is the degree of convergence or divergence of light rays achieved by a lens. It equals the reciprocal of the focal length in metres, . Its SI unit is the dioptre (D).

    1 mark

  2. The focal length is negative, so the lens is a concave (diverging) lens.

    ½ mark

  3. ½ mark

  4. D

    ½ mark

  5. A concave lens always forms a virtual, erect, diminished image on the same side as the object. Hence the magnification is positive (+).

    ½ mark

Answer: Concave (diverging) lens; P = −10 D; magnification is positive

Where the marks are

  • Correct definition of power (1 mark)
  • Identifies concave/diverging lens (½ mark)
  • Formula P = 1/f(m) = 100/f(cm) (½ mark)
  • P = −10 D (½ mark)
  • Sign of magnification positive (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 paper2023 board paper, set 31/2/1: 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.