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When white snow sparkles in different colors

Single ice crystals are transparent. This characteristic makes snow appear white and under the right circumstances even sparkle colorful.

Walking through a sunny winter wonderland, with every step you will notice dazzling white or even colorful sparkles. This sparkling reminds of polished pieces of glass or diamonds. The comparison between snow and glass is more accurate than you might have assumed before. Snow is made out of ice crystals and an ice floe is very similar to a glass plate. Both are transparent, so light passes through without noticeable absorption. In both cases a small part of the light is reflected at the interface to air. If you smash a glass plate into very fine pieces or granules, this transparency will get lost and it will appear white – exactly like ice crystals in the form of snow.

Looking through a single smooth ice floe objects behind are fairly clearly recognized with the exception of slight displacements due to refractions. However, placing small plates with different orientation on top of each other, they slowly get non transparent. A uniformly blended color is the result because you see an overlapping of the light on different places and objects. 

In a similar way, ice crystals of snowflakes loose their transparency. Only a few centimeters of snow cover up the ground and appears as a dazzling white in bright sunshine. In such a case “ice crystals being accumulated randomly”, there is no more difference between reflection and refraction. This interaction is called scattering. In comparison to the wavelength of the light these tiny ice crystals are still fairly large. Therefore, there is mainly forward scattering and no interference effects like you see in thin layers of soap bubbles or insect wings (see “Himmlische Sphären”, Spektrum June 2016, P.44 and “Lebendige Juwelen”, Spektrum May 2016, P.42). The angle of the deflected light is small, and thus a large quantity of ice crystals is needed to change its direction noticeably and be able to emerge from the layer of snow. Just because ice is transparent to its greatest possible extent and ice crystal absorb extremely little light, this phenomena actually happens – and the snow cover takes on its familiar intensive white color.

Merely thicker layers of snow and ice have a bluish shimmer, as we know it of crevasses and icebergs. In the range of wavelength of red light a slightly stronger absorption seems to take place. This coloring, however, must not be confused with the blueness of snow fields in shady areas. In this case white sunlight is blocked and mainly the blue light of the sky is reflected. 

In order to experience the initially mentioned bright and colorful sparkles, special circumstances have to exist. Low temperature and freshly fallen snow are favorable. Crystals will be lying on top in a loose and unscathed manner. Hexagonal types are especially suitable which are already present in the atmosphere due to colorful reflections of sun dogs. Because of their shape they behave like 60° prisms. The light enters at one of the surfaces into the crystal and exits on the opposite side at a refraction index of 1.31 with an angle range of approx. 22° to 46° between sun and observer (see diagram above).

The smallest angle the white sun light leaves a crystal, is between 21.7° (red, 656 nanometer wavelength) and 22.5° (violet, 400 nanometer). The greatest intensity is found close to this minimum: Counting all rays leaving the ice platelets in all possible directions most of them are deflected at an approximate angle of 22°. Therefore, a light flash is coming from this direction. 

Consequently, it depends on the observer’s position, if the light flash can be seen at all and which color it has. The hexagonal prisms are only one example for this effect; differently formed crystals can also deflect sunlight in a similar way and split it in single colors of the spectrum.

Although these light flashes seem to be blinding for the human eye, it is difficult to record them photographically. If the focus is on the snow surface and the exposure time is set on automatic, the flashes are often completely overexposed, outshine the surroundings and loose their color. In addition, they are hardly to be seen on the photograph as the surface of the snow is already shining in a bright white. Reducing the exposure time the colorful reflections can be seen perfectly, to be sure, but the snow surface is underexposed and appears unreal dark. In this situation a slight defocusing is recommended. By doing this, these tiny little color dots are projected as a larger surface and thus, more visible. Unfortunately, the rest of the snow surface is blurred and out of focus (see above picture).

Under certain conditions new ice crystals develop on older snow surface: the melting process during the day and renewed freezing at night cause a new formation of ice crystals which send out light flashes at bright sunshine. They are mainly produced by reflection of the sunlight at well-situated facets and therefore, appear basically white. Light transmission with color splitting due to refraction does not occur on relatively smooth surfaces. 

A surface crust as a result of numerous melting and freezing processes leads to a different collective light phenomena. These unstructured large crystal surfaces have the same effect like closely arranged mini pieces of mirrors. They reflect the low light of the sun in winter in a similar way as the uneven surface of the ocean or a lake. A sparkling ray of light emerges towards the sun, which is also known as “sword of the sun”. Changing one’s position crossways these rays will move along accordingly. Previous reflections dissolve and new ones start flashing – an impressing spectacle.    

http://www.spektrum.de/

H. Joachim Schlichting was director of the Institute for Didactics of Physics at the University of Münster. He was awarded the Archimedes Prize for physics in 2013.