ScienceCBSE Class 10 · Board exam 2025, set 31/2/1 · 3 marks · Short answer

CBSE Class 10 Science 2025: question 31

Set in the CBSE Class 10 board exam of 2025, paper 31/2/1, 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

A convex lens forms an 8.0 cm long image of a 2.0 cm long object which is kept at a distance of 6.0 cm from the optical centre of the lens. If the object and the image are on the same side of the lens, find (i) the nature of the image, (ii) the position of the image, and (iii) the focal length of the lens.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/2/1.

Hints

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

Hint 1

An image on the same side as the object, formed by a convex lens, is of which kind?

Hint 2

Use magnification m = h'/h = v/u to find v, then use the lens formula. Watch the signs.

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.

(i) The image is on the same side as the object and is larger than the object (8.0 cm vs 2.0 cm). So the image is virtual and erect (and magnified).
1 mark
(ii) Given $h = + 2.0$ cm, $h^{'} = + 8.0$ cm (erect), $u = - 6.0$ cm.
$m = \frac{h ^{'}}{h} = \frac{v}{u} \Rightarrow \frac{8.0}{2.0} = \frac{v}{- 6.0}$
½ mark
$v = 4 \times (- 6.0) = - 24$ cm. The image is 24 cm from the lens, on the same side as the object.
½ mark
(iii) Lens formula: $\frac{1}{v} - \frac{1}{u} = \frac{1}{f} \Rightarrow \frac{1}{- 24} - \frac{1}{- 6} = \frac{1}{f}$
½ mark
$\frac{1}{f} = - \frac{1}{24} + \frac{4}{24} = \frac{3}{24} = \frac{1}{8} \Rightarrow f = + 8$ cm. The focal length of the convex lens is 8 cm.
½ mark

Answer: (i) Virtual, erect (magnified); (ii) v = −24 cm (24 cm on the object side); (iii) f = +8 cm

Where the marks are

Nature: virtual and erect (1 mark)
Uses m = h'/h = v/u with correct substitution (½ mark)
v = −24 cm (½ mark)
Uses lens formula with correct values (½ mark)
f = 8 cm (½ mark)

Revise the chapter

Chapter: Light – Reflection and Refraction (notes, formulas, where marks are lost and every past question)
Topic: Lens Formula, Magnification and Power of a Lens

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 31/2/1: 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.

CBSE Class 10 Science 2025: question 31

Hints

Step-by-step solution

Revise the chapter

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