At sunset, the Sun appears red, yet the sky around it may still appear slightly bluish. Briefly explain why these two colours appear simultaneously in the sky at that moment.
Generated by claude-sonnet-4-6 · 2026-06-26 01:09 · grounding rag
Model Answer
At sunset, sunlight travels a longer path through the atmosphere. Most of the blue light (shorter wavelength) is scattered away by fine particles, leaving mainly red and orange light to reach our eyes — making the Sun appear red.
However, some blue light scattered sideways from other parts of the atmosphere still reaches our eyes, making the surrounding sky appear slightly bluish.
Source: Chapter 10, Section 10.6.2 — Scattering of Light
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Explanation
- The key concept is scattering of light: shorter wavelengths (blue) scatter more, longer wavelengths (red) scatter least.
- At sunset, the longer atmospheric path means nearly all blue is scattered out of the direct line of sight → red Sun.
- Blue light scattered at angles from other overhead/surrounding air still enters the eye → slightly blue sky.
- Examiners expect both effects to be linked explicitly to scattering and wavelength. One mark for each colour's explanation.