AI-generated practice question — model-generated for extra practice, not a previous-year CBSE board question.
Using $n_m = \dfrac{c}{v}$, speed of light in diamond $= \dfrac{3 \times 10^8}{2.42} \approx 1.24 \times 10^8 \ \text{m s}^{-1}$.
Source: Chapter 9, Section 9.3.2 The Refractive Index
The examiner expects the formula $n = c/v$ rearranged to $v = c/n$, substitution of values, and the final answer with correct unit. Writing the formula first earns the method mark; the correct numerical answer earns the answer mark (in a 2-mark version). For 1 mark, the final answer with unit is essential.