ScienceCBSE Class 10 · Board exam 2024, set 31/2/1 · 2 marks · Very short answer

CBSE Class 10 Science 2024: question 24(b)

Set in the CBSE Class 10 board exam of 2024, paper 31/2/1, section B, question 24(b), for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q24(b) · 2 marks · Very short answerLight – Reflection and Refraction

Define the term absolute refractive index of a medium. A ray of light enters from vacuum to glass of absolute refractive index 1.5. Find the speed of light in glass. The speed of light in vacuum is $3 \times 1 0^{8}$ m/s.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 31/2/1.

Hints

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

Hint 1

Refractive index compares the speed of light in two places.

Hint 2

Use $n = c / v$ and make $v$ the subject.

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.

Definition: The absolute refractive index of a medium is the ratio of the speed of light in vacuum to the speed of light in that medium: $n_{m} = \frac{c}{v}$ .
1 mark
Given: $c = 3 \times 1 0^{8}$ m/s and $n_{g} = 1.5$ . We need $v$ in glass. Using $n_{g} = \frac{c}{v}$ , we get $v = \frac{c}{n _{g}}$ .
½ mark
$v = \frac{3 \times 1 0 ^{8}}{1.5} = 2 \times 1 0^{8}$ m/s. So the speed of light in glass is $2 \times 1 0^{8}$ m/s.
½ mark

Answer: Absolute refractive index = speed of light in vacuum ÷ speed of light in the medium; speed of light in glass = $2 \times 1 0^{8}$ m/s

Where the marks are

Correct definition of absolute refractive index (1 mark)
Writes $n = c / v$ and rearranges it to $v = c / n$ (½ mark)
Correct answer $2 \times 1 0^{8}$ m/s with unit (½ mark)

Revise the chapter

Chapter: Light – Reflection and Refraction (notes, formulas, where marks are lost and every past question)
Topic: Refraction of Light and Refractive Index

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 paper2024 board paper, set 31/2/1: every question, by section.

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CBSE Class 10 Science 2024: question 24(b)

Hints

Step-by-step solution

Revise the chapter

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