Science — CBSE Class 10 board question

(a)

From the diagram, the ray bends towards the normal entering Medium B, then bends away from the normal entering Medium C, and the ray in C is parallel to the ray in A (like a glass slab).

(i) Speed of light: Speed in A > Speed in C > Speed in B

(ii) Optical density: Medium B is optically densest, Medium A is rarest, and C is intermediate. So: $n_B > n_C > n_A$

Justification (speed): The refractive index of medium 2 w.r.t. medium 1 is $n_{21} = \dfrac{v_1}{v_2}$. A higher refractive index means lower speed. Since $n_B$ is highest, light travels slowest in B. Since $n_C > n_A$, light travels faster in A than in C.

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(b)

(i) When the ray falls obliquely on Medium B, which is optically denser than both A and C:

• At the A–B boundary: ray bends towards the normal (entering denser medium).
• At the B–C boundary: ray bends away from the normal (entering rarer medium).
• The emergent ray in C is not parallel to the incident ray in A (since A and C have different optical densities).

(Diagram: ray bends toward normal at top surface, bends away at bottom, with different angles in A and C.)

(ii) When the ray falls normally (perpendicularly) on Medium B:

• The ray passes straight through without bending at either surface.
• There is no change in direction at any boundary.

(Diagram: straight, undeviated ray passing through all three media.)

Source: Chapter 9, Sections 9.3, 9.3.1, 9.3.2

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• For part (a), the key is reading the diagram: the ray bends toward the normal going into B (so B is densest), and the emergent ray in C is parallel to A (as in a glass slab), which means C's density is intermediate. Link speed and optical density using $n = v_1/v_2$.
• For part (b)(i), since A and C have different refractive indices, the emergent ray is not parallel to the incident ray — unlike a standard glass slab where both outer media are the same (air). Examiners expect this distinction.
• For part (b)(ii), a normally incident ray has angle of incidence = 0°, so angle of refraction = 0° — it goes straight through. This is a standard result from Snell's law.