ScienceCBSE Class 10 · Board exam 2025, set 31/1/1 · 2 marks · Very short answer

CBSE Class 10 Science 2025: question 24

Set in the CBSE Class 10 board exam of 2025, paper 31/1/1, section B, question 24, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q24 · 2 marks · Very short answerLight – Reflection and Refraction

An object is placed at a distance of 60 cm from a concave lens of focal length 30 cm. Use lens formula to find the position of the image formed in this case.

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

Hints

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

Hint 1

Write down the sign convention first. Is the focal length of a concave lens positive or negative?

Hint 2

Use $\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$ and rearrange it to make $\frac{1}{v}$ the subject.

Hint 3

Take care with the signs: $\frac{1}{v} = \frac{1}{f} + \frac{1}{u}$ , with both $f$ and $u$ negative.

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.

Using the sign convention for a concave lens: object distance $u = - 60$ cm and focal length $f = - 30$ cm. A concave lens is diverging, so $f$ is negative.
Lens formula: $\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$
½ mark
Substituting the values: $\frac{1}{- 30} = \frac{1}{v} - \frac{1}{- 60}$ , so $\frac{1}{v} = - \frac{1}{30} - \frac{1}{60}$
½ mark
$\frac{1}{v} = \frac{- 2 - 1}{60} = \frac{- 3}{60} = - \frac{1}{20}$ , so $v = - 20$ cm.
½ mark
The image is formed 20 cm from the lens, on the same side as the object. The negative sign shows this. The image is virtual and erect.
½ mark

Answer: v = -20 cm: image 20 cm from the lens, on the same side as the object (virtual, erect)

Where the marks are

States the lens formula $\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$ (½ mark)
Substitutes $u = - 60$ cm and $f = - 30$ cm correctly (½ mark)
Obtains $v = - 20$ cm and states the image position (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 paper2025 board paper, set 31/1/1: every question, by section.

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CBSE Class 10 Science 2025: question 24

Hints

Step-by-step solution

Revise the chapter

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