Answer the following. (a) Write the expression for the magnification produced by a lens in terms of object distance and image distance. (b) A 4 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 20 cm. Calculate the size of the image formed, if the distance of the object from the lens is 10 cm.

ScienceCBSE Class 10 · Board exam 2026, set 31/2/1 · 3 marks · Short answer

CBSE Class 10 Science 2026: question 35

Set in the CBSE Class 10 board exam of 2026, paper 31/2/1, section C, question 35, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q35 · 3 marks · Short answerLight – Reflection and Refraction

Answer the following.

(a)
Write the expression for the magnification produced by a lens in terms of object distance and image distance.
0 marks
(b)
A 4 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 20 cm. Calculate the size of the image formed, if the distance of the object from the lens is 10 cm.
0 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 31/2/1.

Hints

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

Hint 1

Use the New Cartesian sign convention: object distance is negative, focal length of a convex lens is positive.

Hint 2

Apply $\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$ to get $v$ , then $m = v / u$ .

Hint 3

Image height = magnification × object height.

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.

(a) Magnification produced by a lens: $m = \frac{image distance}{object distance} = \frac{v}{u}$
1 mark
(b) Given (sign convention): $h_{o} = + 4$ cm, $f = + 20$ cm (convex lens), $u = - 10$ cm.
Lens formula: $\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$ , so $\frac{1}{v} = \frac{1}{f} + \frac{1}{u}$
½ mark
$\frac{1}{v} = \frac{1}{20} + \frac{1}{- 10} = \frac{1 - 2}{20} = - \frac{1}{20}$ , so $v = - 20$ cm (virtual image on the same side as the object).
½ mark
$m = \frac{v}{u} = \frac{- 20}{- 10} = + 2$ .
Since $m = \frac{h _{i}}{h _{o}}$ : $h_{i} = m \times h_{o} = 2 \times 4 = 8$ cm. The image is 8 cm tall, virtual and erect.
1 mark

Answer: (a) $m = v / u$ ; (b) $v = - 20$ cm, $m = + 2$ , image height = 8 cm (virtual, erect)

Where the marks are

Writes $m = v / u$ (1 mark)
Uses lens formula with correct signs (½ mark)
Finds $v = - 20$ cm (½ mark)
Finds $m = 2$ and image size 8 cm (1 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 paper2026 board paper, set 31/2/1: every question, by section.

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CBSE Class 10 Science 2026: question 35

Hints

Step-by-step solution

Revise the chapter

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