A small rainbow in the sky is often caused by iridescence, which appears as spectral colors in clouds.
Understanding Iridescence
Iridescence isn't a typical rainbow caused by rain; rather, it's a result of diffraction, where sunlight bends and spreads out when it passes through tiny water droplets or ice crystals in the atmosphere.
How It Forms:
- Cloud Type: Iridescence typically occurs in high-altitude clouds like cirrus or lenticular clouds.
- Tiny Particles: These clouds are made of particularly small ice crystals or water droplets.
- Light Interaction: When sunlight hits these tiny particles, it's diffracted, meaning the light waves are bent and scattered. This separates the white sunlight into its constituent spectral colors.
Key Characteristics:
- Appearance: The colors are often pastel and can appear as bands, patches, or edges on clouds.
- Size: These iridescent patches are typically much smaller and more localized compared to a full rainbow.
- Location: Iridescence forms near clouds, specifically those containing tiny ice crystals or water droplets.
Why It's Not a Regular Rainbow
- Rainbows need water droplets from rain to form. They form when light is refracted (bent going into a water droplet), internally reflected (bouncing off the inside of the droplet) and refracted again (bent coming out of the droplet)
- Iridescence appears without rain and due to diffraction rather than refraction and reflection.
In Summary:
| Feature | Iridescence | Rainbow |
|---|---|---|
| Formation | Diffraction of light by tiny droplets/ice crystals | Refraction and reflection of light by raindrops |
| Appearance | Small, often pastel colored patches on/near clouds | Large arc of spectral colors |
| Requirement | Tiny water droplets or ice crystals in clouds | Rain droplets |
Thus, a small rainbow in the sky seen in clouds is more accurately called iridescence and is caused by a different optical phenomenon than a typical rainbow. As the reference points out, iridescence forms high in the sky and near clouds because that is where tiny ice crystals and water droplets are found that can diffract sunlight into spectral colors.
