Q1. [3] § Chapter Introduction § 10.4 DISPERSION OF WHITE LIGHT BY A GLASS PRISM
Draw a ray diagram to show the formation of a rainbow in the sky. On this diagram mark A – where dispersion of light occurs, B – where internal reflection of light occurs and C – where refraction of light occurs. List two necessary conditions to observe a rainbow.
Previously asked in CBSE board exam
2024 31/2/1 Q31(b)
Generated by claude-sonnet-4-6 · 2026-06-15 06:54 · grounding rag
Model Answer
Ray Diagram:
```
Sunlight → [Enter droplet]
A (Dispersion - refraction at entry)
↘
B (Internal reflection at back of droplet)
↙
C (Refraction at exit)
→ Dispersed colours to observer's eye
```
Draw a circular water droplet. Mark:
- A – at the point where sunlight enters the droplet (dispersion occurs here)
- B – at the back surface inside the droplet (total internal reflection)
- C – at the point where light exits the droplet (second refraction)
Two necessary conditions to observe a rainbow:
- The Sun should be behind the observer (rainbow forms in the direction opposite to the Sun).
- Rain/water droplets must be present in the atmosphere in front of the observer.
Source: Chapter 10, Section 10.4 – Dispersion of White Light by a Glass Prism
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Explanation
- The textbook states that water droplets refract and disperse (A) → internally reflect (B) → refract again (C). Examiners expect all three labels correctly placed on the droplet diagram.
- The two conditions come directly from the text: Sun behind you, and water droplets (rain) in front. Many students forget to mention the Sun's position — this is a common mark-losing error.
- Since this is a diagram-based question, even a rough but correctly labelled sketch earns marks. Neatness and correct labelling of A, B, C are key.
If a question refers to an image, map, graph or diagram that is not shown here, find the actual CBSE question paper on the CBSE website:
cbse.gov.in.