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The students in a class took… (Class 10 Science 2025, Q39)

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ScienceCBSE Class 10 · Board exam 2025, set 31/4/1 · 4 marks · Case-based

CBSE Class 10 Science 2025: question 39

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

The students in a class took a thick sheet of cardboard and made a small hole in its centre. Sunlight was allowed to fall on this small hole and they obtained a narrow beam of white light. A glass prism was taken and this white light was allowed to fall on one of its faces. The prism was turned slowly until the light that comes out of the opposite face of the prism appeared on the nearby screen. They studied this beautiful band of light and concluded that it is a spectrum of white light.

  1. (i)

    Give any one more instance in which this type of spectrum is observed.

    1 mark
  2. (ii)

    What happens to white light in the above case?

    1 mark
  3. (iii)(A)

    List two conditions necessary to observe a rainbow.

    2 marks

  4. OR

    (iii)(B)

    Draw a ray diagram to show the formation of a rainbow. Mark on it, points (a), (b) and (c) as given below : (a) Where dispersion of light occurs. (b) Where light gets reflected internally. (c) Where final refraction occurs.

    2 marks

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

Think of a natural, coloured band seen in the sky after rain.

Hint 2

Splitting of white light into its colours has a particular name.

Hint 3

In a raindrop, light is refracted on entering, reflected at the back, and refracted again on leaving; where must the Sun be for you to see it?

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. (i) A rainbow in the sky after a shower is a natural spectrum of sunlight.

    1 mark

  2. (ii) The white light undergoes dispersion. Its component colours bend through different angles in the prism (violet the most, red the least), so it splits into the band VIBGYOR.

    1 mark

  3. (iii)(A) Condition 1: tiny water droplets must be present in the atmosphere, for example just after rain.

    1 mark

  4. (iii)(A) Condition 2: the Sun must be behind the observer, with the observer facing the droplets.

    1 mark

  5. (iii)(B) Diagram: draw a circle for a raindrop and a ray of sunlight striking its upper-left surface. Inside the drop, show the light splitting into red and violet rays that travel to the back surface. There they reflect, then emerge from the lower-left surface heading downward, with red and violet separated, towards the observer.

    ½ mark

  6. (iii)(B) Mark (a) at the point where sunlight enters the drop: refraction and dispersion occur here.

    ½ mark

  7. (iii)(B) Mark (b) at the back surface of the drop: total internal reflection occurs here.

    ½ mark

  8. (iii)(B) Mark (c) at the point where light leaves the drop: the final refraction occurs here, and the colours reach the eye.

    ½ mark

Answer: (i) Rainbow; (ii) Dispersion of white light into its seven colours; (iii)(A) Water droplets in the air, and the Sun behind the observer; (iii)(B) Raindrop diagram: (a) entry surface – dispersion, (b) back surface – internal reflection, (c) exit surface – final refraction.

Where the marks are

  • Names a natural spectrum such as a rainbow (1 mark)
  • States that dispersion of white light takes place (1 mark)
  • Water droplets present in the atmosphere (1 mark)
  • Sun behind the observer (1 mark)
  • Correct raindrop ray diagram (½ mark) with points a, b and c correctly marked (½ × 3 marks)

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 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.