By illuminating hexagonal glass crystals which spin about one or two axes (simultaneously), together with my wife, we recreate one of nature’s most beautiful atmospheric optics phenomena: ice halos.
While everyone knows about rainbows, atmospheric ice halos do not share the same popular recognition, despite them being no less imposing! After we became aware of the phenomenon a few years ago, we have ever since been astonished by the frequency with which one may observe halo displays once you know when and where to look for them.
Their formation mechanism is somewhat similar in the sense that it is a special type of refraction caustic, i.e. an effect of light concentration in certain directions mediated by the bending of light through transparent bodies of certain shapes. For rainbows, the shape explaining their appearance is a sphere, whereas for halos hexagonal prisms are responsible. Each type of geometric light path through the prismatic ice crystals corresponds to a particular halo seen by the observer on the skye sphere. The variety of observable halo phenomena large, owing to the complex geometry of the refracting hexagonal ice crystals and their possible in-air orientations they can take!
Motivated by the search for a way to artificially reproduce these optical phenomena we finally, after several attempts, came up with an effective modular device:
With a little bit of soldering, small DC-motors, LiPo battery packs and small chunks of hexagonal glass rods (used as light homogenization rods in the LED industry, e.g. obtainable through optics suppliers), we spin glass crystals about the appropriate axis. Placing the machines in the center of a custom spherical projection screen, we can now recreated these imposing atmospheric optics phenomena in our living room (http://photonicsdesign.jimdo.com/physics-at-home/)!
The corresponding scientific articles we have written can be found here: http://photonicsdesign.jimdo.com/publications/
General information about halos can be found here:
More info: photonicsdesign.jimdo.com
A typical natural ice halo display, showing two colored sundogs (parhelia) 22° left and right from the sun.
Another natural display showing the tangent arc and the circular 22° ice halo.
The most complex halo machine consists of a glass crystal (mimicking the ice crystals) glued to a micro-motor, which in turn spins on a second motor. The single crystal thus emulates a large ensemble of many crystals with different ordered orientations.
The machines are placed, one by one, in the center of a projection screen covered with fluorescent spray paint.
In operation and upon illumination by laser-light, a projection appears on the surface.
Each machine (we have built 4 different modules) produces a different set of arcs, bows and spots.
Superposing the individual images produces a a beautifully complex pattern of light on the spherical projection screen.
The artificial glass halos have corresponding natural ice halo counterparts and can be identified accordingly.
The light deflected by the spinning crystal in the spheres center traces out certain paths on the sphere, which in their sum generate the halo appearance.
A hexagonal plate crystal spun about 2 axis causes this analogon of the so-called Lowitz-halo
A column-like hexagonal crystal spun around a single axis causes this analogon of the natural Parry halo phenomenon.
A hexagonal plate crystal spinning about its symmetry single axis generates this parhelic circle analogon with embedded parhelia (“sundogs”, the bright spots).
A column-like crystal spun about two axis generates this analogon of the tangential arc halo (2s exposure).
A short exposure (1/100s) image reveals the light paths which constitute the final tangent arc projection.
Crystals not only deflect light, but also splits the colors of white light. This causes the natural counterparts to be vividly colored.
Hexagonal column-like ice crystals like shaped like this glass crystal cause the natural phenomena. The image shows the glass crystal with a laser ray going through it.
Looking at a single hexagonal glass crystal illuminated by white light from above, beautiful and rich patterns emerge.