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SKIING WITH LAMBERT -- SNOW, ILLUMINATION AND MACHINE VISION (continued)
2. Optical Material -- How Snow Turns White
ice crystals | air | refracted | reflected | surface | scattering | white color | resolution | "diffuse" reflection | color saturation
As we all know, snow is consisted of small ICE CRYSTALS (fig. 2_a).
fig. 2_a: Various Ice Crystals
figure taken from
"Mein grosses Bildlexikon", Augsburg 2003, ISBN 3-8289-6013-8, page 128
with courtesy of Weldon Owen Publishing, Sydney NSW 2060, Australia.
But an important component of snow is the AIR inbetween.
If we would pack the ice densely (without AIR spaces), it would look like glass:
clear transparent with a light hue of greenish-blue.
On the other hand, if we replace the air by water, only dark-grey mud is left over.
Why is snow white in spite of the different color of ice?
Light striking a solid lump of ice is in part REFRACTED and in part REFLECTED; quantitatively described by the law of Snellius (Snellius 1591 - 1626).
An important quantity in this law is the refractive index n .
Normally, the refractive index of an optical material - here, ice - differs from the index of the surrounding medium - here, AIR.
The more it differs, the bigger the refractive effect and the reflective effect.
Otherwise, icicles would not be so well-visible (fig. 2_b).
All this happens at the ice SURFACE.
fig. 2_b: Icicles in sunlight
Now in a volume of snow, very many small ice SURFACE elements are sitting closely spaced.
Every single one of them positioned at a randomly varying angle.
They all refract and reflect an incoming light ray -- SCATTERING its energy into all directions of space.
The refractive index difference (necessary for refraction and reflection) is nearly independent of the wavelength (in the realm of visible wavelengths).
And so the multitude of ice grains, every one of them sending light of all wavelengths into several directions, make the WHITE COLOR of snow.
Why don't we see lots of sparkling facets, but only a smooth white surface? --
The eye's RESOLUTION simply is not good enough.
We can either take a microscope, and the facets will shine up.
Or we wait for a frosty night. Water, condensing out of the air, builds up bigger ICE CRYSTALS on the snow surface.
In the morning sun, lots of tiny stars will shine up in the white (fig. 2_c).
fig. 2_c: sparkling snow surface
Size comparison: Coin = 10 Eurocent, 20 mm dia.
Snow surface about 1 week old.
Incoming rays of light are diffused by snow into several random directions.
That is why we call this a "DIFFUSE" REFLECTION, as opposed to the "specular" reflection of a mirror.
Concerning "WHITE COLOR":
There are people who insist that black, grey, and white are not really colors.
Because their COLOR SATURATION is zero.
(On "COLOR SATURATION" see my essay "C O L O R : SENSATION -- PHYSICS -- TECHNOLOGY, para. 1.3.1)
Expressed in a different way, they are hues which are defined by a totally even spectral distribution of light.
I prefer to call them colors, though, maybe, a special family of colors.
By the way, the same mechanism of causing diffuse white reflection works in several white materials:
salt and sugar in grains or powders, small glass beads, powder of TiO2 and of BaSO4, fog and clouds, emulsions, watery or oily foams, and paper.
If you know some white material that works in a different way, please tell me: Dietmar.Dowe@t-online.de
3. Hole in the White -- How Snow Turns Blue
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Last modified Nov. 30st, 2009