Science — CBSE Class 10 board question

(a) Medium B is optically denser than medium A. This is because the ray of light bends towards the normal while entering medium B from medium A, which happens when light travels from a rarer (optically less dense) medium to a denser medium.

(b) The refractive index of medium B with respect to medium A is:

$$n_{BA} = \frac{\text{Speed of light in medium A}}{\text{Speed of light in medium B}} = \frac{V_A}{V_B}$$

Source: Chapter 9, Section 9.3.2 — The Refractive Index

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• (a): The key rule is: light bends towards the normal → going into denser medium; bends away → going into rarer medium. Always justify with this rule.
• (b): Refractive index of medium 2 w.r.t. medium 1 = speed in medium 1 ÷ speed in medium 2. Don't flip it. Here medium A is "1" and medium B is "2", so it's $V_A / V_B$.