Science — CBSE Class 10 board question

A rainbow is a natural spectrum formed in the sky after a rain shower. It is caused by dispersion of sunlight by tiny water droplets in the atmosphere. The water droplets act as small prisms. Sunlight entering a droplet is refracted and dispersed, then undergoes total internal reflection inside the droplet, and is refracted again while emerging. Due to dispersion, different colours (VIBGYOR) reach the observer's eye at different angles. A rainbow is always formed in the direction opposite to the Sun.

Ray Diagram:

```
Sunlight → [Water Droplet]
↗ Refraction (dispersion)
↘ Internal Reflection
↘ Refraction (exit)
→ Violet (inner arc) to Red (outer arc) → Observer's Eye
```

(Labelled diagram should show: incident sunlight, refraction at entry, internal reflection, refraction at exit, and emergent colours R, O, Y, G, B, I, V)

Source: Chapter 10, Section 10.4 Dispersion of White Light by a Glass Prism

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• 3 marks = definition/cause (1 mark) + process inside droplet (1 mark) + diagram (1 mark).
• Key terms examiners look for: dispersion, refraction, internal reflection, water droplets act as prisms, opposite direction to Sun, VIBGYOR.
• The diagram must be labelled with at least: incident ray, refraction, internal reflection, emergent colours. A neat, labelled sketch earns the diagram mark even if not perfectly drawn.
• Do not confuse the order: Red is on the outer arc, Violet on the inner arc of the rainbow.