PhysicsCBSE Class 12 · Board exam 2024, set 55/1/1 · 2 marks · Very short answer

CBSE Class 12 Physics 2024: question 19

Set in the CBSE Class 12 board exam of 2024, paper 55/1/1, section B, question 19, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q19 · 2 marks · Very short answerRay Optics and Optical Instruments

An object is placed 30 cm in front of a concave mirror of radius of curvature 40 cm. Find the (i) position of the image formed and (ii) magnification of the image.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 55/1/1.

Hints

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

Hint 1

First find the focal length from the radius of curvature.

Hint 2

Use $1/ v + 1/ u = 1/ f$ with the Cartesian signs for a concave mirror.

Hint 3

For a mirror, magnification is $m = - v / u$ .

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) By the sign convention, $u = - 30$ cm and $f = - R /2 = - 20$ cm. Mirror formula: $\frac{1}{v} + \frac{1}{u} = \frac{1}{f}$ gives $\frac{1}{v} + \frac{1}{- 30} = \frac{1}{- 20}$ .
½ mark
$\frac{1}{v} = - \frac{1}{20} + \frac{1}{30} = - \frac{1}{60}$ , so $v = - 60$ cm. The image is real and lies 60 cm in front of the mirror.
½ mark
(ii) Magnification: $m = - \frac{v}{u} = - \frac{( - 60 )}{( - 30 )}$ .
½ mark
$m = - 2$ . The image is inverted and twice the size of the object.
½ mark

Answer: (i) v = −60 cm (real, 60 cm in front of mirror); (ii) m = −2

Where the marks are

Uses the mirror formula with $f = - 20$ cm, $u = - 30$ cm (½ mark)
Obtains v = −60 cm (½ mark)
Writes $m = - v / u$ (½ mark)
Obtains m = −2 (½ mark)

Revise the chapter

Chapter: Ray Optics and Optical Instruments (notes, formulas, where marks are lost and every past question)

Practise this chapterBoard-style questions from Ray Optics and Optical Instruments, one at a time, marked against the answer key and marking points.Back to the full paper2024 board paper, set 55/1/1: every question, by section.

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CBSE Class 12 Physics 2024: question 19

Hints

Step-by-step solution

Revise the chapter

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