Q1. [2] deep thorough-understanding
Kerosene floats on water, showing that its mass density is less than that of water. Yet a light ray bends more when entering kerosene from air than when entering water from air. How do you account for this apparent contradiction?
Generated by claude-sonnet-4-6 · 2026-06-26 01:14 · grounding rag
Model Answer
Mass density and optical density are two different properties. The refractive index of a medium depends on its optical density (i.e., the speed of light in it), not its mass density. Kerosene has a refractive index of 1.44, which is higher than that of water (1.33). This means light travels slower in kerosene than in water, so it bends more when entering kerosene from air. There is no contradiction — a medium of lower mass density can still be optically denser.
Source: Chapter 9, Section 9.3.2 (The Refractive Index)
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Explanation
- Examiners expect you to clearly distinguish mass density from optical density / refractive index.
- Key fact to cite: kerosene $n = 1.44$ > water $n = 1.33$ (from Table 9.3).
- The core idea: bending of light depends on the speed of light in the medium (optical density), NOT on how heavy the substance is.
- This is explicitly noted in the textbook: "an optically denser medium may not possess greater mass density."