The Halo

single ice crystal — 60° prism, minimum deviation

sky — halos around the sun

Halos are the rainbow's frozen twin. Where a bow is born in round water drops, a halo is born in hexagonal ice crystals. Sunlight enters one face, refracts, and leaves another. Through faces inclined at 60° the light piles up at its angle of least deviation — about 21.8° — and rings the sun.

Single crystal

Ray incidence i₁ 40°

Sky

Sun elevation 15°

Skin

Drag on the crystal view to set the incidence angle. The D(i) plot marks the minimum — the bright halo ray. Halo colour is true wavelength→sRGB, so red sits on the inner edge (it deviates least). Raise the sun and the sundogs drift outward; the circumzenithal arc vanishes above 32°.