Angles of minimum deviation for ice prisms

The solution: D = f(A) when n = 1.32 is the boundary between the white and purple regions.

The angle of minimum deviation for an ice crystal with a prism angle of 60° is 22°. This minumum deviation gives rise to the common 22° halo when randomly oriented hexagonal prisms are present. If pyramid shaped crystals are present a 9° halo can arise when the prisom angle os 29 8°.