Explain with a labelled ray diagram how a rainbow is formed in the sky after a rain shower. Name all the optical phenomena that occur inside a water droplet and describe the role each plays in producing the coloured arc.
Generated by claude-sonnet-4-6 · 2026-06-26 01:08 · grounding rag
Model Answer
Rainbow Formation:
A rainbow is formed after rain by dispersion of sunlight through tiny water droplets suspended in the atmosphere. It is always formed opposite to the Sun.
Ray Diagram:
```
Sunlight → [Water Droplet]
/ Refraction (entry)
↓
Internal Reflection
↓
\ Refraction (exit)
→ Dispersed colours (Violet to Red) → Observer's eye
```
(Labelled diagram should show: incident white light, refraction at entry, internal reflection at back surface, refraction at exit, with violet bending most and red least.)
Optical Phenomena and their roles:
- Refraction (at entry): Disperses white sunlight into its seven component colours (VIBGYOR).
- Internal Reflection: Reflects the dispersed light back inside the droplet toward the observer.
- Refraction (at exit): Further separates the colours, making the coloured arc visible to the observer.
Source: Chapter 10, Section 10.4
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Explanation
- Examiners expect all three phenomena named: refraction (twice) and internal reflection — missing any costs marks.
- The diagram must be labelled (incident ray, refraction, internal reflection, emergent colours).
- Note violet deviates most, red least — this is why colours appear as a band/arc.
- Keep diagram simple; a neat single-droplet diagram is sufficient for 3 marks.