Skip to content

A person allowed a narrow… (Class 10 Science 2025, Q39)

questionbuddy.in/q/cbse-10-science-2025-31-2-1-q39-person-allowed-narrow-beam-white

ScienceCBSE Class 10 · Board exam 2025, set 31/2/1 · 4 marks · Case-based

CBSE Class 10 Science 2025: question 39

Set in the CBSE Class 10 board exam of 2025, paper 31/2/1, 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-basedThe Human Eye and the Colourful World

A person allowed a narrow beam of white light from the sun to enter a dark room through a small aperture and placed a glass prism in its path in such a manner that the beam falls on the face AB of the prism as shown in the figure. A screen S is placed on the other side of the prism, facing AC. On turning the prism slowly, a beautiful band of colours is obtained on the screen. It is the spectrum of sunlight.

  1. (a)

    Name the phenomenon due to which a prism splits the incident white light into a band of colours.

    1 mark
  2. (b)

    State the reason of getting a band of seven colours in the above case.

    1 mark
  3. (c)(i)

    Explain with the help of a labelled ray diagram, an experimental arrangement to show the recombination of the spectrum of white light.

    2 marks

  4. OR

    (c)(ii)

    Draw a labelled ray diagram to show the formation of a rainbow.

    2 marks

Figure in the paper: A triangular glass prism with vertices A (top), B (bottom left) and C (bottom right). A narrow white light ray labelled 'White light' with an arrow travels up-right and falls on face AB. A vertical screen S is placed to the right of the prism, facing face AC. See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/2/1.

Hints

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

Hint 1

What is the splitting of white light into its colours called?

Hint 2

Do red and violet light bend by the same amount in glass?

Hint 3

For recombination, think of using a second prism that undoes the first; for a rainbow, follow light into a raindrop, off its back surface and out again.

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) Dispersion of light.

    1 mark

  2. (b) The different colours of white light bend through different angles with respect to the incident ray as they pass through the prism. Red bends the least and violet the most, so the colours separate into a band.

    1 mark

  3. (c)(i) Arrangement: Take two identical glass prisms, A and B. Place prism B upside down (inverted) relative to prism A, next to it.

    1 mark

  4. (c)(i) Diagram and explanation: Label white light falling on prism A. Show it splitting into a spectrum between the prisms, labelled R (red) to V (violet). Show the colours entering inverted prism B and emerging as a single beam labelled white light falling on a screen. The second prism bends the colours back by equal and opposite amounts, so they recombine into white light.

    1 mark

  5. (c)(ii) Rainbow diagram: Draw a spherical raindrop and label incoming sunlight (white light) striking its upper part. Show the light refracting on entering and splitting into colours (dispersion). Show it reflecting internally at the back of the drop and refracting again as it leaves.

    1 mark

  6. (c)(ii) Labels: Mark the emerging red and violet rays, and an observer standing with the sun behind them. Label the steps: refraction and dispersion, internal reflection, refraction. Put arrows on all rays.

    1 mark

Answer: (a) Dispersion of light. (b) Different colours bend through different angles in the prism. (c)(i) Two identical prisms, the second inverted, recombine the spectrum into white light. (c)(ii) Raindrop diagram showing refraction with dispersion, internal reflection and refraction, with the observer facing away from the sun.

Where the marks are

  • Dispersion of light (1 mark)
  • Different colours bend through different angles (1 mark)
  • (c)(i) Two identical prisms, one inverted (1 mark)
  • (c)(i) Labelled diagram showing the spectrum recombining into white light (1 mark)
  • (c)(ii) Raindrop with refraction, dispersion and internal reflection (1 mark)
  • (c)(ii) Labels, rays with arrows and observer (1 mark); ½ mark deducted if arrows are missing

Revise the chapter

Practise this chapterBoard-style questions from The Human Eye and the Colourful World, one at a time, marked against the answer key and marking points.Back to the full paper2025 board paper, set 31/2/1: 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.