Absolute refractive index of water and glass is 4/3 and 3/2 respectively. If the speed of light in glass is 2×10^8 m/s, the speed of light in water is: (A) 9/4 m/s (B) 7/3 m/s (C) 16/9 m/s (D) 9/8 m/s

ScienceCBSE Class 10 · Board exam 2025, set 31/3/1 · 1 mark · MCQ

CBSE Class 10 Science 2025: question 11

Set in the CBSE Class 10 board exam of 2025, paper 31/3/1, section A, question 11, for 1 mark. Try it first; the hints and our step-by-step solution are below.

Q11 · 1 mark · MCQLight – Reflection and Refraction

Absolute refractive index of water and glass is $\frac{4}{3}$ and $\frac{3}{2}$ respectively. If the speed of light in glass is $2 \times 1 0^{8}$ m/s, the speed of light in water is:

(A) $\frac{9}{4}$ m/s
(B) $\frac{7}{3}$ m/s
(C) $\frac{16}{9}$ m/s
(D) $\frac{9}{8}$ m/s

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/3/1.

Hints

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

Hint 1

The speed of light in a medium is inversely proportional to its refractive index.

Hint 2

Use $v_{w} / v_{g} = n_{g} / n_{w}$ .

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.

Since $n = c / v$ , we have $\frac{v _{w}}{v _{g}} = \frac{n _{g}}{n _{w}} = \frac{3/2}{4/3} = \frac{9}{8}$ .
½ mark
$v_{w} = \frac{9}{8} \times 2 \times 1 0^{8} = \frac{9}{4} \times 1 0^{8} = 2.25 \times 1 0^{8}$ m/s. Option (A) shows $\frac{9}{4}$ but leaves out the factor $1 0^{8}$ .
½ mark

Answer: $\frac{9}{4} \times 1 0^{8}$ m/s (none of the printed options is exactly correct; (A) is intended)

Where the marks are

Full credit of 1 mark to all candidates as per the scheme (1 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 paper2025 board paper, set 31/3/1: every question, by section.

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CBSE Class 10 Science 2025: question 11

Hints

Step-by-step solution

Revise the chapter

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