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| Light ray and ice crystal | |
| The next image illustrates how light goes through an ice crystal.
Light ray from a light source (white light from up) hit an hexagonal ice crystal which is floating in the atmosphere horizontally (also called as plate orientation). In this case a small part of the light ray reflects from ice crystal surface toward right (in the image). This is ray for parhelic circle. Because light is only reflected its color is white.
Most of the light ray will go in the ice crystal through entry face. Then the light ray hit the other sideface of the crystal. Most of the light refract through this face and exit the ice crystal. The entry and this exit face act like an icy prism which wedge angle is 60°. This prism causes dispersion of the light and we will see a colorful parhelion 22° from the sun.
A small part of light which hit the second face reflects. This light will hit a third face and most of it will refract through it. In this case, the inner reflection will eliminate refraction effect of the refraction and outgoing light is white. However, it has experienced parallel shift of colors which can be seen in this computer generated image. This light will be also part of parhelic circle.
Light ray may experience more complex ray paths through ice crystal than in this simplified sample. For some rare halos several, some times more than ten, internal reflections are needed. It is easy to see why that kind of halos may be rare. If there is any impurity or flaws in the ice crystals the ray path may be impossible.
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