Frost Kaleidoscope ~ Ice crystals on a window imaged by Jesper Gr�nne in Denmark.   ©Jesper Gr�nne, shown with permission.

Jesper used a 85mm f/1,2 lens and a reversed 50mm outside it to give an enlarged macro image. The image covers just a few millimetres.

The crystals grow across the glass from tiny nuclei and surface imperfections. Unlike fern frost these crystals are small and quickly jostle others before they can get larger. The growth nuclei were either very close together or conditions rendered crystal nucleation and growth was rapid.

The secret of the magical colours is that Jesper imaged the frost between two crossed polarising filters.

Ice is birefringent or double refracting (see earlier OPOD ). Placing a polarising filter each side of the crystals reveals the colours that result from interference between the two birefringent rays.

Colours are also produced by thin film interference. Distinguish between them by rotating the camera polariser. birefringent colours then change, those from thin films do not.