ICE HALO
Ice prisms at minimum deviation — the 22° ring around the sun, red on the inside, exact to 1e-12.
Use the simulation above to change the variables and play through the guided stages. The explanation below describes the default starting values; the simulation updates its explanation as you experiment.
Setup
High cirrus is made of tiny hexagonal ice prisms, tumbling in every orientation. Sunlight passes through millions of them at once — and somehow the chaos resolves into a clean, sharp ring, always at the same radius.
Least bend
A prism bends light by an amount that depends on how the ray enters — except at one angle, the symmetric passage, where the deviation reaches a minimum. Randomly-oriented crystals pile their light up at exactly that minimum.
The radius
For the sixty-degree faces of an ice hexagon, the minimum is about twenty-two degrees. Nothing from this ray family reaches inside it, so the sky brightens abruptly at the ring edge.
Reversed color
Ice disperses light too, but the geometry flips the order: red deviates least, so the ring is red on the inside and bluish outside — the mirror image of a raindrop’s bow. The color is faint because the crystals are not perfectly aligned.
Audit
The same minimum-deviation formula that measures a prism’s glass measures the sky’s ice. Traced symmetrically it agrees to twelve digits, the twenty-two and forty-six degree halos lock, and the whole apparition is one closed expression.