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A convex lens can form a (i)… (Class 10 Science 2023, Q30b)

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ScienceCBSE Class 10 · Board exam 2023, set 31/5/1 · 3 marks · Short answer

CBSE Class 10 Science 2023: question 30(b)

Set in the CBSE Class 10 board exam of 2023, paper 31/5/1, section C, question 30(b), for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q30(b) · 3 marks · Short answerLight – Reflection and Refraction

A convex lens can form a (i) real, inverted and magnified image as well as (ii) virtual, erect and magnified image of an object. If the focal length of the lens is 10 cm, what should be the range of the object distance in both cases ? Draw ray diagrams to justify your answer.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 31/5/1.

Hints

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

Hint 1

With f = 10 cm, where are F and 2F located?

Hint 2

For a magnified real image the object is beyond F but not beyond 2F.

Hint 3

A convex lens gives a virtual image only when the object is closer than F.

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. (i) Real, inverted, magnified image: the object must lie between F and 2F, i.e. 10 cm < object distance < 20 cm.

    ½ mark

  2. (ii) Virtual, erect, magnified image: the object must lie between the optical centre O and F, i.e. object distance < 10 cm.

    ½ mark

  3. Ray diagram (i): draw a convex lens with F₁, 2F₁ on the left and F₂, 2F₂ on the right. Place object AB between F₁ and 2F₁. A ray parallel to the axis refracts through F₂. A ray through O goes straight on. They meet beyond 2F₂, giving a real, inverted, enlarged image A′B′ below the axis. Mark arrows on all rays.

    1 mark

  4. Ray diagram (ii): place object AB between O and F₁. The ray parallel to the axis refracts through F₂, and the ray through O goes straight on. These rays diverge, so extend them backwards as dotted lines. They meet on the same side as the object, giving a virtual, erect, enlarged image A′B′ behind the object. Mark arrows on all rays.

    1 mark

Answer: (i) Between 10 cm and 20 cm (between F and 2F); (ii) less than 10 cm (between O and F)

Where the marks are

  • Range for real magnified image: 10 cm < u < 20 cm (½ mark)
  • Range for virtual magnified image: u < 10 cm (½ mark)
  • Correct ray diagram for object between F and 2F (1 mark)
  • Correct ray diagram for object between O and F, with arrows on rays (1 mark; ½ deducted if ray directions are not marked)

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 31/5/1: 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.