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A narrow beam XY of white… (Class 10 Science 2023, Q25a)

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ScienceCBSE Class 10 · Board exam 2023, set 31/1/1 · 2 marks · Very short answer

CBSE Class 10 Science 2023: question 25(a)

Set in the CBSE Class 10 board exam of 2023, paper 31/1/1, section B, question 25(a), for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q25(a) · 2 marks · Very short answerThe Human Eye and the Colourful World

A narrow beam XY of white light is passing through a glass prism ABC as shown in the diagram :

Trace it on your answer sheet and show the path of the emergent beam as observed on the screen PQ.
Name the phenomenon observed and state its cause.

Figure in the paper: A triangular glass prism ABC with apex A at top and base BC horizontal. A ray XY from the lower left (X) strikes face AB at point Y. A vertical screen PQ stands to the right of the prism, with P at top and Q at bottom. See it in the official paper.

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

Hints

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

Hint 1

Think about what happens when sunlight passes through a glass prism and falls on a wall.

Hint 2

Which colour bends the least, and which bends the most towards the base?

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. Ray diagram (described in words):

    • Draw the prism ABC and the incident white ray XY striking face AB at Y.
    • Inside the prism, the ray splits into a fan of seven coloured rays.
    • These rays bend towards the base BC. Red bends the least and violet bends the most.
    • The rays leave through face AC, bending further towards the base.
    • On the screen PQ they form a band of colours. From top (P side) to bottom (Q side) the order is R, O, Y, G, B, I, V: red at the top and violet at the bottom.
    • Arrows should be shown on all rays.
    1 mark

  2. Phenomenon: This is the dispersion of white light, that is, the splitting of white light into its seven component colours (the spectrum).

    ½ mark

  3. Cause: Light of different colours (different wavelengths) travels at different speeds in glass. So each colour bends through a different angle with respect to the incident ray. Red is deviated the least and violet the most.

    ½ mark

Answer: The beam splits into a spectrum, with red at the top and violet at the bottom on PQ (VIBGYOR read from the bottom up). The phenomenon is dispersion, caused by different colours bending through different angles.

Where the marks are

  • Correct diagram: splitting inside the prism, red least and violet most deviated, colours labelled on screen PQ (1 mark)
  • Names dispersion of white light (½ mark)
  • Cause: different colours or wavelengths bend through different angles (½ mark)

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