ScienceCBSE Class 10 · Board exam 2025, set 31/4/1 · 3 marks · Short answer

CBSE Class 10 Science 2025: question 31

Set in the CBSE Class 10 board exam of 2025, paper 31/4/1, 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

If we want to obtain a virtual and magnified image of an object by using a concave mirror of focal length 18 cm, where should the object be placed? Use mirror formula to determine the object distance for an image of magnification +2 produced by this mirror to justify your answer.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/4/1.

Hints

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

Hint 1

Recall the ray-diagram cases for a concave mirror. Which object position gives a virtual image?

Hint 2

Use $m = - v / u$ to write $v$ in terms of $u$ , then substitute into the mirror formula with a negative $f$ .

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.

A concave mirror forms a virtual, erect and magnified image only when the object is placed between the pole P and the focus F, that is, at less than 18 cm from the mirror.
1 mark
Mirror formula: $\frac{1}{v} + \frac{1}{u} = \frac{1}{f}$ , with $f = - 18$ cm.
½ mark
Magnification $m = - \frac{v}{u} = + 2$ , so $v = - 2 u$ .
1 mark
Substituting: $\frac{1}{- 2 u} + \frac{1}{u} = \frac{1}{- 18}$
$\frac{1}{2 u} = - \frac{1}{18}$
$u = - 9$ cm, and then $v = + 18$ cm (a virtual image behind the mirror).
The object is 9 cm in front of the mirror, which is less than $f = 18$ cm. It lies between P and F, which justifies the answer.
½ mark

Answer: Object between P and F (less than 18 cm from the mirror); $u = - 9$ cm.

Where the marks are

States that the object is between P and F, i.e. less than 18 cm (1 mark)
Writes the mirror formula with $f = - 18$ cm (½ mark)
Uses $m = + 2$ to get $v = - 2 u$ (1 mark)
Obtains $u = - 9$ cm (½ mark)

Revise the chapter

Chapter: Light – Reflection and Refraction (notes, formulas, where marks are lost and every past question)

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 31/4/1: 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

More questions from Light – Reflection and Refraction