2D Ice Halo Ray Tracer — Hexagonal Prism Refraction

Crystal geometry

Effective prism angle

Ray

Incident angle 40°

Refractive index n 1.310

Show ray path & angles

Live readout

Deviation, minimum deviation and halo radius update instantly in the HUD above the diagram.

Why 22°? A ray through the 60° faces of a hexagonal ice prism (n≈1.31) has a minimum possible deviation of about 21.8°. Rays pile up at that angle because the deviation curve is flat there (dD/di=0), so the sky is brightest right at the halo's inner edge and dark just inside it — nothing can deviate less.

Deviation (this ray)
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Minimum deviation
— °

Halo radius
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Ray diagram — hexagonal ice-crystal cross-section

Halo brightness vs. deviation angle