Experimental Demonstrations of Column Halos

Experimental Equipment | Photo: Michael Großmann

Did you ever think about showing in a practical way, how ice-crystal halos come to be? An individual halo like, e.g., a sundog is easy to demonstrate, but producing halo phenomena in such a way is more difficult, as a multitude of crystal shapes and orientations are involved.

Raypath sundog | Photo: Michael Großmann

In 2011, I committed myself to the task of generating halos in a darkened room by optomechanical devices, for which the term „halomators“ was coined. Any one of several models can hold an artificial crystal and make it rotate in certain ways. When light impinges on this crystal, different kinds of halos are produced by the possible reflective and refractive raypaths. By means of the so-called sky transform it can be shown that a single crystal produces the same kind of display in a laboratory environment as a multitude of such crystals in nature.

For the experiment shown here, a horizontally rotating column crystal was at first doubly oriented, then only singly.
In the beginning (with a rectangular face of the crystal staying in the horizontal plane of the rotation too) the Parry orientation tape halos very nicely showed, including the corresponding reflection halo like heliac arc, subhelic arc and the parhelic circle. When the crystal was made to rotate about its symmetry axis, the upper tangent arc also appeared.

Natural photographs, however, can show still more halos from oriented columns. This led me to further improve my halomators, for which my profession as an industry mechanic proved helpful. By 2016 I could document numerous other kinds of halos.

“Halomator IV”| Photo: Michael Großmann

The following images are stacks of photographs of halos obtained from singly and doubly oriented columns. They show the following types:

– upper tangent arc
– concave Parry arc
– convex Parry arc
– heliac arc
– subhelic arc
– subanthelic arc
– Tricker’s anthelic arc
– Wegener‘s anthelic arc
– Hasting’s arc
– parhelic circle

Remark: I had to use acrylic (PMMA) as a workable material for my crystals, which does not match the refractive index of ice. Therefore, most deflection angles change. Also, those refraction halos produced in ice by the 90° prism angle do not show at all, because of total internal reflection. (Ways out of this limitation are being investigated by other experimenters and myself, and will then possibly be reported.) Despite of these shortcomings, I found out in many demonstrations, that most people grasp the optics behind a certain halo much better than from a sketch or computer simulation.

Author: Michael Großmann, Bilfingen, Germany

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