Double rainbows are a rare optical and meteorological phenomenon where two concentric arcs of colour appear in the sky. The primary rainbow is brighter with red on the outer edge and violet on the inner, while the secondary rainbow is fainter and has the colours reversed. This spectacular display is a classic example of light refraction, reflection, and dispersion in water droplets.
Key Facts for Prelims
| Aspect | Details |
|---|---|
| Definition | A double rainbow occurs when sunlight is both refracted and reflected twice inside raindrops, producing two distinct arcs of colour. |
| Primary Rainbow | Formed when sunlight refracts, reflects once inside the droplet, and disperses into its component colours. Bright and vivid, with red on top and violet at the bottom. |
| Secondary Rainbow | Formed when sunlight reflects twice inside the droplet before emerging. It appears fainter and has reversed colour order (violet on top and red at the bottom). |
| Physical Processes Involved | Refraction (bending of light), Reflection (bouncing within the droplet), and Dispersion (splitting of white light into its component wavelengths). |
| Angle of Formation | – Primary rainbow: ~42° from the line opposite the Sun. – Secondary rainbow: ~50–53° from the line opposite the Sun. |
| Visibility | Best seen when the Sun is low in the sky and there are raindrops opposite the Sun. |
| Why Secondary Arc is Fainter | Each internal reflection results in light loss; thus, the second reflection produces weaker intensity and reversed colours. |
| “All Rainbows are Double” Concept | Theoretically, all rainbows have a secondary arc, but it is often too faint to be visible due to light intensity and viewing conditions. |
| Scientific Significance | Demonstrates fundamental optics principles—especially the interaction of light with spherical water droplets. |
Conceptual Explanation
Sunlight entering a raindrop slows and bends (refraction), splits into colours (dispersion), reflects inside the droplet, and exits (refraction again). In double rainbows, a second internal reflection occurs, reversing the order of colours and shifting the arc’s position. Each reflection weakens intensity, hence the secondary arc appears dimmer.
Observational Conditions
- Occurs during rainfall with sunlight coming from behind the observer.
- The observer faces the part of the sky opposite the Sun.
- Clear, low-sun conditions after or during light rain enhance visibility.
Reference: The Hindu
