Since they are caused by the interaction of light with
ice crystals, light pillars belong to the family of
halos. The
crystals responsible for light pillars usually consist of flat,
hexagonal plates, which tend to orient themselves more or less horizontally as they fall through the air. Each flake acts as a tiny
mirror which reflects light sources that are appropriately positioned below it (see drawing), and the presence of flakes at a spread of altitudes causes the reflection to be elongated vertically into a column. The larger and more numerous the crystals, the more pronounced this effect becomes. More rarely, column-shaped crystals can cause light pillars as well. In very cold weather, the ice crystals can be suspended near the ground, in which case they are referred to as
diamond dust. Unlike a
light beam, a light pillar is not physically located above or below the light source. Its appearance as a vertical line is an
optical illusion, resulting from the collective
reflection off the ice crystals; but only those that are in the common
vertical plane, direct the light rays towards the observer (See drawing). This is similar to viewing a light source on a body of water. Ripples on the surface of the water reflect the light source in many directions, and those that happen to be aimed at the viewer, combine to form a bright line pointing toward the light source. == See also ==