A light ray enters from medium A to medium B as shown in the figure. (a) Which one of the two media is denser w.r.t. other medium? Justify your answer. (b) If the speed of light in medium A is v_a and in medium B is v_b, what is the refractive index of medium B with respect to A?

ScienceCBSE Class 10 · Board exam 2023, set 31/6/1 · 2 marks · Very short answer

CBSE Class 10 Science 2023: question 25(a)

Set in the CBSE Class 10 board exam of 2023, paper 31/6/1, 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 answerLight – Reflection and Refraction

A light ray enters from medium A to medium B as shown in the figure.

(a)
Which one of the two media is denser w.r.t. other medium? Justify your answer.
1 mark
(b)
If the speed of light in medium A is $v_{a}$ and in medium B is $v_{b}$ , what is the refractive index of medium B with respect to A?
1 mark

Figure in the paper: A horizontal boundary line with a vertical dashed normal at the point of incidence. A ray in medium 'A' (lower left) with an arrow travels up to the boundary and refracts into medium 'B' (upper right) bending away from the normal. Labels: 'Medium B' above, 'Medium A' below. See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 31/6/1.

Hints

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

Hint 1

Compare the angle the ray makes with the normal in each medium.

Hint 2

Refractive index of B with respect to A is the ratio of speeds, with the speed in A on top.

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.

(a) Medium B is optically denser than medium A.
½ mark
Reason: on entering medium B, the ray bends towards the normal, so $∠ r < ∠ i$ . Light slows down in a denser medium.
½ mark
(b) $n_{B A} = \frac{speed of light in A}{speed of light in B} = \frac{v _{a}}{v _{b}}$
1 mark

Answer: (a) Medium B (ray bends towards normal, ∠r < ∠i); (b) n_BA = v_a / v_b

Where the marks are

Identifies medium B as denser (½ mark)
Justifies: ray bends towards normal / ∠r < ∠i (½ mark)
Writes n_BA = v_a/v_b (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 paper2023 board paper, set 31/6/1: 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.

CBSE Class 10 Science 2023: question 25(a)

Hints

Step-by-step solution

Revise the chapter

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