Riya is reading about refraction and comes across a table of refractive indices. She notes: air (1.0003), water (1.33), kerosene (1.44), crown glass (1.52) and diamond (2.42). She also reads that refraction is caused by the change in speed of light as it moves from one medium to another.
(i) In which of the listed media does light travel at the lowest speed? Justify your answer. [1 mark]
(ii) Riya notices that kerosene has a higher refractive index than water. Does this mean kerosene is also more mass-dense than water? Explain. [1 mark]
(iii) A light ray travels from water into crown glass. Does it bend towards or away from the normal? Give a reason. [1 mark]
(iv) If the speed of light in vacuum is 3 × 10⁸ m s⁻¹, calculate the speed of light in crown glass. [1 mark]
Generated by claude-sonnet-4-6 · 2026-06-26 01:13 · grounding rag
Model Answer
(i) Light travels at the lowest speed in diamond (n = 2.42). Higher refractive index means lower speed of light in the medium ($n = c/v$), so the highest n gives the lowest v.
(ii) No. Optical density (refractive index) is not the same as mass density. Kerosene is optically denser than water (higher n), but its mass density is actually less than that of water.
(iii) The light bends towards the normal. Crown glass (n = 1.52) is optically denser than water (n = 1.33). When light travels from a rarer to a denser medium, it slows down and bends towards the normal.
(iv)
$$n = \frac{c}{v} \implies v = \frac{c}{n} = \frac{3 \times 10^8}{1.52} \approx 1.97 \times 10^8 \text{ m s}^{-1}$$
Source: Light – Reflection and Refraction, Section 9.3.2
---
Explanation
- (i) The key formula is $n = c/v$; highest n → lowest v. Diamond has n = 2.42, the highest in the table.
- (ii) The textbook explicitly states: "an optically denser medium may not possess greater mass density" and uses kerosene vs water as the exact example. Quote or paraphrase this directly.
- (iii) Compare refractive indices: water (1.33) < crown glass (1.52), so water is rarer. Rarer → denser means bending towards the normal.
- (iv) Straightforward formula application. Show the formula, substitution, and answer with units. $3 \times 10^8 / 1.52 \approx 1.97 \times 10^8$ m s⁻¹ is the expected result.