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SKIING WITH LAMBERT -- SNOW, ILLUMINATION AND MACHINE VISION (continued)
3. Hole in the White -- How Snow Turns Blue
shadow | blue sky | hole in the white snow | greenish-blue light | path lengths in ice | cyan | additive color-mixing
Now we know why snow looks white.
But consider the image of a snowy landscape (e.g. fig. 3_a).
fig. 3_a: snow surface partly in shadow
("lange Wand", Ischgl/Austria)
You see that snow in sunlight looks white but -- what's that? -- in the SHADOW, it looks blue.
Well, every good photographer knows this effect: SHADOWs are blue.
While in direct sunlight, the illuminant spectrum of the sun is effective, in the SHADOW the BLUE SKY illuminates everything with a blueish light.
It's so normal that we normally don't notice it.
Our visual system senses the surface colors only; it compensates for illuminant colors as far as possible.
Now take a ski pole and poke a HOLE IN THE WHITE SNOW surface (fig. 3_b).
fig. 3_b: blue hole in white snow
In the hole, a mysterious GREENISH-BLUE LIGHT ("CYAN") appears.
What is that? Light from the BLUE SKY?
Apparently not. First, because it is a different hue -- too green.
And second: we can also drill a horizontal hole into the snow (fig. 3_c), so as to avoid blue skylight shining into the hole.
fig. 3_c: snow surface with blue horizontal hole
And still, the mysterious CYAN is shining at the inner snow surface.
Though the outer surface of snow is white, the inner one looks blue; why that? --
Light rays, hitting the snow surface, are split into thousands of weeker sub-rays by multiple reflection and refraction (as pointed out in
Many of theses sub-rays leave the snow surface after a short "random walk" and constitute its white appearance.
But in this "erratic" or random walk (fig. 3_d), some other sub-rays penetrate deeper into the snow.
fig. 3_d: light ray is refracted and reflected in snow
Adding up their PATH LENGTHS IN ICE, you'll see that considerable ice-lengths can modify the light color (like a thick block of ice would do).
And through air gaps and reflections, enough white light also penetrates the snow so that the resulting color is "whiter" than that of massive ice.
So, by an ADDITIVE COLOR-MIXING process, the blue-green color of ice and the white sunlight constitute the CYAN-colored light in a snow hole.
For additional information
- on the color "CYAN" see my essay
C O L O R : SENSATION -- PHYSICS -- TECHNOLOGY, para. 2.3 "Colored Light"
and same essay, para. 3.2 "Mixing Colors Subtractively"
- on "additive color-mixing" see my essay
C O L O R : SENSATION -- PHYSICS -- TECHNOLOGY, para. 3.1 "Mixing Colors Additively"
4. Lost Structure -- How Snow Turns Invisible
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Last modified Dec. 1st, 2009