Define the term absolute refractive index of a medium. A ray of light enters from vacuum to glass of absolute refractive index 1.5. Find the speed of light in glass. The speed of light in vacuum is 3 × 10⁸ m/s.
Generated by claude-sonnet-4-6 · 2026-06-15 06:51 · grounding rag
Model Answer
Absolute Refractive Index: The absolute refractive index of a medium is the ratio of the speed of light in vacuum (or air) to the speed of light in that medium.
$$n_m = \frac{c}{v}$$
Calculation:
$$n_g = \frac{c}{v} \Rightarrow v = \frac{c}{n_g} = \frac{3 \times 10^8}{1.5} = 2 \times 10^8 \text{ m/s}$$
The speed of light in glass is $2 \times 10^8$ m/s.
Source: Chapter 9, Section 9.3.2 — The Refractive Index
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Explanation
- Definition (1 mark): State clearly that it is the ratio of speed of light in vacuum to speed of light in the medium. The formula $n_m = c/v$ is expected.
- Calculation (1 mark): Rearrange the formula to find $v = c/n$, substitute values, and state the answer with correct units.
- Do not confuse absolute refractive index (medium w.r.t. vacuum) with relative refractive index (one medium w.r.t. another).