Rainbow Formation
An optical meteorological phenomenon caused by reflection, refraction, and chromatic dispersion of sunlight inside raindrops.
Governing formula
θ_primary ≈ 42° (Red outer, violet inner)
SI unit
Degrees (°)
In depth
Sunlight enters a raindrop, refracts, reflects once off the back wall, and refracts out. Differential refraction disperses light into a circular arc centered opposite the Sun at 42°.
Examples in the real world
A bright rainbow arc forming across the sky when sunlight shines through falling rain behind the observer.