Observe the following diagram and compare (i) speed of light and (ii) optical densities of the three media A, B and C. Also give justification for your answer of any one of the two cases in terms of refractive indices of A, B and C. (a) Observe the diagram and compare (i) speed of light and (ii) optical densities of media A, B and C; justify one case in terms of refractive indices. (b) Redraw the path of a ray of light through the three media, if the ray of light starting from medium A falls on the medium B (i) obliquely and the optical density of medium B is made more than that of A and C; (ii) the ray falls normally from medium A to medium B.

ScienceCBSE Class 10 · Board exam 2025, set 31/5/1 · 5 marks · Long answer

CBSE Class 10 Science 2025: question 36(A)

Set in the CBSE Class 10 board exam of 2025, paper 31/5/1, section D, question 36(A), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q36(A) · 5 marks · Long answerCompetency-basedLight – Reflection and Refraction

Observe the following diagram and compare (i) speed of light and (ii) optical densities of the three media A, B and C. Also give justification for your answer of any one of the two cases in terms of refractive indices of A, B and C.

(a)
Observe the diagram and compare (i) speed of light and (ii) optical densities of media A, B and C; justify one case in terms of refractive indices.
0 marks
(b)
Redraw the path of a ray of light through the three media, if the ray of light starting from medium A falls on the medium B (i) obliquely and the optical density of medium B is made more than that of A and C; (ii) the ray falls normally from medium A to medium B.
0 marks

Figure in the paper: A rectangular block labelled Medium A (above), Medium B (inside the block) and Medium C (below). A ray enters from the top-left, bends toward the normal while passing into Medium B, then bends away from the normal on exiting into Medium C at the bottom right. See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/5/1.

Hints

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

Hint 1

Look at each boundary and ask: does the ray bend, and if so, towards or away from the normal?

Hint 2

No bending means equal refractive indices. Bending away from the normal means the ray has entered a rarer medium.

Hint 3

Use $n = c / v$ : a larger refractive index means slower light and a denser medium.

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)(i) Speed of light: The ray goes from A into B without bending, so light travels at the same speed in A and B. On passing from B into C it bends away from the normal, so light travels faster in C than in A and B. Thus $v_{A} = v_{B} < v_{C}$ .
1 mark
(a)(ii) Optical density: A and B have the same optical density. C is optically rarer (less optically dense) than A and B.
1 mark
Justification using refractive index: A ray is not deviated between media of equal refractive index, so $n_{A} = n_{B}$ . A ray bends away from the normal when it goes into a medium of lower refractive index, so $n_{C} < n_{B}$ . Since $n = \frac{c}{v}$ , a lower refractive index means a higher speed of light and a lower optical density. Hence $n_{A} = n_{B} > n_{C}$ .
1 mark
(b)(i) Oblique incidence, B denser than A and C: Draw the ray striking the A–B surface at an angle. At the A–B surface it bends towards the normal inside B. At the B–C surface it bends away from the normal into C. Draw normals (dotted) at both points of incidence and mark arrows on the ray.
1 mark
(b)(ii) Normal incidence: Draw the ray falling perpendicular to the A–B surface. It goes straight through B and C without bending (angle of incidence = 0°, so angle of refraction = 0°).
1 mark

Answer: (a) Speed: $v_{A} = v_{B} < v_{C}$ ; optical density: A = B > C, because $n_{A} = n_{B} > n_{C}$ and $n = c / v$ . (b)(i) The ray bends towards the normal on entering B and away from the normal on entering C. (ii) The ray passes straight through all three media without deviation.

Where the marks are

Speed of light is the same in A and B and greater in C (1 mark)
Optical density is the same for A and B and lower for C (1 mark)
Justifies using refractive indices: no bending from A to B means $n_{A} = n_{B}$ ; bending away from the normal into C means $n_{C}$ is lower; $n \propto 1/ v$ (1 mark)
Ray diagram for oblique incidence: towards the normal in B, away from the normal in C (1 mark)
Ray diagram for normal incidence: ray passes undeviated (1 mark)

Revise the chapter

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

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 31/5/1: every question, by section.

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CBSE Class 10 Science 2025: question 36(A)

Hints

Step-by-step solution

Revise the chapter

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