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A student took three concave… (Class 10 Science 2023, Q39)

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ScienceCBSE Class 10 · Board exam 2023, set 31/6/1 · 4 marks · Case-based

CBSE Class 10 Science 2023: question 39

Set in the CBSE Class 10 board exam of 2023, paper 31/6/1, section E, question 39, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q39 · 4 marks · Case-basedCompetency-basedLight – Reflection and Refraction

A student took three concave mirrors of different focal lengths and performed the experiment to see the image formation by placing an object at different distances with these mirrors as shown in the following table.

Case No.Object-distanceFocal length
I45 cm20 cm
II30 cm15 cm
III20 cm30 cm

Now answer the following questions:

  1. (a)

    List two properties of the image formed in Case I.

    0 marks
  2. (b)

    In which one of the cases given in the table, the mirror will form real image of same size and why?

    0 marks
  3. (c)

    Name the type of mirror used by dentists. Give reason why do they use such type of mirrors.

    0 marks
  4. (c)

    Look at the table and identify the situation (object distance and focal length) which resembles the situation in which concave mirrors are used as shaving mirrors? Draw a ray diagram to show the image formation in this case.

    0 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 31/6/1.

Hints

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

Hint 1

For each case, find and compare it with the object distance.

Hint 2

Same size image: where must the object be relative to C?

Hint 3

Shaving and dental mirrors need an enlarged, erect image. Which case has the object closer than the focus?

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.

  1. (a) In Case I, cm, so cm. The object at 45 cm lies beyond C. The image therefore forms between F and C and is (any two): real, inverted and diminished.

    1 mark

  2. (b) Case II. Here cm, so cm. The object distance is 30 cm, which means the object is placed at the centre of curvature C.

    ½ mark

  3. A concave mirror forms a real, inverted image of the same size at C when the object is placed at C.

    ½ mark

  4. (c) [Option A] Dentists use concave mirrors.

    1 mark

  5. When a tooth is placed very close to a concave mirror (between P and F), the mirror forms an erect and enlarged (magnified) image. This lets the dentist see the tooth clearly.

    1 mark

  6. (c) [Option B] Case III (object distance 20 cm, focal length 30 cm) resembles a shaving mirror. Here the object lies between the pole P and the focus F, so the image is virtual, erect and enlarged.

  7. Ray diagram (in words): Draw the concave mirror with its principal axis and mark P, F (30 cm) and C (60 cm). Place the object AB at 20 cm, between P and F. Ray 1 from A runs parallel to the axis and reflects through F. Ray 2 from A is directed towards the mirror along the line joining C to A and is reflected back along the same path. The reflected rays diverge, so extend them backwards with dotted lines behind the mirror. They meet at A′, giving an erect, enlarged, virtual image A′B′ behind the mirror. Arrows must show the direction of the rays.

    (Option B carries the same 2 marks as Option A: 1 for identifying Case III, 1 for the diagram.)

Answer: (a) Real, inverted, diminished (any two). (b) Case II – the object is at C (u = 2f = 30 cm). (c) A: concave mirror, as it gives an erect, enlarged image of a tooth held close to it. (c) B: Case III (u = 20 cm, f = 30 cm), object between P and F gives a virtual, erect, enlarged image (ray diagram).

Where the marks are

  • (a) Any two of real, inverted, diminished (½ + ½ marks)
  • (b) Case II (½ mark), with the reason that the object is at C since 30 cm = 2f (½ mark)
  • (c) Names the concave mirror (1 mark) and gives the reason: erect, enlarged image when the object is close (1 mark)
  • OR (c) Identifies Case III (1 mark) and draws a correct ray diagram with ray directions marked (1 mark; ½ deducted if arrows are missing)

Revise the chapter

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 paper2023 board paper, set 31/6/1: 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.