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A student placed a candle… (Class 10 Science 2025, Q31)

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ScienceCBSE Class 10 · Board exam 2025, set 31/6/1 · 3 marks · Short answer

CBSE Class 10 Science 2025: question 31

Set in the CBSE Class 10 board exam of 2025, paper 31/6/1, section C, question 31, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q31 · 3 marks · Short answerCompetency-basedLight – Reflection and Refraction

A student placed a candle flame at different distances from a convex lens and focused its image on a screen. He recorded his observation in tabular form as given below :

S.No.Distance of flame from the lens (cm)Distance of the image from the lens
1−90+18
2−60+20
3−40+24
4−30+30
5−24+40
6−20+60
7−18+90
8−12+120

Analyse the observation table and on the basis of your analysis only, answer the following questions (without doing any calculations) :

  1. (a)

    What is the focal length of the convex lens used ? Give reason to justify your answer.

    0 marks
  2. (b)

    Which one of the sets of observations is not correct and why ?

    0 marks
  3. (c)

    Draw ray diagram to show image formation for any correct set of observation.

    0 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/6/1.

Hints

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

Hint 1

Look for the row where object and image distances are equal. Where must the object be for that to happen?

Hint 2

Once you know f, check which object distance is less than f. What kind of image does a convex lens form then?

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) Focal length cm.

    ½ mark

  2. (a) Reason: in observation 4, object distance = image distance = 30 cm. This happens only when the object is at and the image is at . So cm, giving cm.

    ½ mark

  3. (b) Observation No. 8 ( cm, cm) is not correct.

    ½ mark

  4. (b) Reason: 12 cm is less than cm, so the object is between the optical centre and the focus. A convex lens then forms a virtual, erect image on the same side as the object, which cannot be caught on a screen. So cannot be cm; it should be negative.

    ½ mark

  5. (c) Ray diagram for observation 4 (object at , 30 cm):

    • Draw the convex lens on the principal axis. Mark , on the left and , on the right, 15 cm and 30 cm from O.
    • Place an upright object AB at .
    • A ray from B parallel to the principal axis refracts through .
    • A ray from B through the optical centre O goes on undeviated.
    • The rays meet below the axis at B′, forming image A′B′ at .
    • The image is real, inverted and the same size as the object. Arrows show the direction of the rays.
    1 mark

Answer: (a) f = +15 cm (u = v = 30 cm means object at 2F); (b) Observation 8, since object within f gives a virtual image on the same side; (c) ray diagram for object at 2F giving a real, inverted, same-size image at 2F′.

Where the marks are

  • Focal length 15 cm (½ mark)
  • Reason: u = v at 2F, so 2f = 30 cm (½ mark)
  • Identifies observation 8 (½ mark)
  • Reason: object lies between O and F, so the image is virtual and on the same side (½ mark)
  • Correct labelled ray diagram for any correct observation (1 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 31/6/1: 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.