The Science of Rainbows: Meteorological Conditions and Optical Physics
Discover how water droplets and sunlight collaborate to create one of nature's most stunning optical phenomena, including double rainbows and supernumeraries.
How Rainbows Form
At its core, a rainbow is a meteorological phenomenon caused by reflection, refraction, and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky.
1. Refraction and Reflection
When sunlight encounters a spherical raindrop, it bends (refracted) as it enters the drop, reflects off the back of the droplet, and bends again as it exits. This processes separates the white sunlight into its component wavelengths.
2. The Critical Angle of 42 Degrees
Because different colors of light bend at slightly different angles, they disperse. Red light exits the droplet at an angle of 42 degrees relative to the incoming sunlight, while violet light exits at 40 degrees. This creates the circular arc shape centered on the anti-solar point.
3. Double Rainbows and Supernumerary Bands
A secondary rainbow forms when light reflects twice inside the raindrop. This causes the colors to reverse (red on the inside, violet on the outside) and reduces brightness. Supernumerary bands are faint, pastel-colored bands on the inner edge of the primary rainbow caused by wave interference.
