If the Sun is Yellow, Why is Snow White?
If snow reflects all light, how does it appear white when the sun is yellowish?
By Douglas Krantz
Question: If the sun is yellow, why is snow white?
Answer: Because the Sky is Blue.
Um, the riddle seems to be nonsense, but really, it does hold a scientific truth.
As the light from the sun travels through the atmosphere, the blue value light is scattered by the moisture. This gives us the yellowish light from the sun we see.
White Snow - Yellow Sun
The snow, indeed, does look to be white. Whereas the real sun doesn't look as yellow as these drawings show, it does have a definite yellow tinge to it, especially when compared to real snow.
Sunlight Above the Atmosphere
Above the atmosphere, the sunlight starts out with a blueish white color. As the light from the sun pierces the atmosphere, it is modified; some of the blue light from the sun is diverted so it no longer comes down to the earth in a straight line.
The snow takes all this light, the yellowish light from the sun and the bluish light from the sky, and recombines it to make white light.
Blue Sky
Moisture in the atmosphere scatters the blue light, and re-scatters the blue light again and again so it seems to come from all directions of the sky. Some of the blue light from the sun is scattered back into space, a little is absorbed by the atmosphere, and in what we observe as blue sky, much of it does get to earth.
What's left of the original white light from the sun is white, minus some of the blue component. To us it looks yellowish.
Size Ratios
The sun is very intense, but it's also a very small portion of the sky.
On the other hand, the blue of the sky isn't all that intense. However, to make up for the lack of intensity, it takes up most of the sky. Added together, all that blue light from all over the sky equals a lot of blue.
This means that as a whole, when added together, the light from the yellow sun and the light from the blue sky provides a total of white light.
The Snow Combines the Light
Snow, of course, is what adds the light together. The light is reflected and refracted by the snow, which makes the snow a combiner of the white light we see.
Question
Hi Douglas,
Doesn't yellow + blue = green?
Anyway, thank you for the informative articles on your website? I greatly appreciate it!
Thank you, D T
Additive Mixing
You're thinking of the subtractive method of mixing colors, like yellow paint mixed with blue paint. With paint, usually, yellow and blue paint gives you green paint.
With snow, however, we have to think of the additive method of mixing colors, like television or computer screens. These systems match the human eye, and snow, just like the screens, is a color additive mixer.
The human eye has a monochrome receptor, and three color receptors. The colors are red, green, and blue. The human eye does not have a yellow receptor. If something is yellow, the color tickles both the red and the green receptors about equally. The brain, then, is what perceives this equal tickling of red and green as yellow.
The sun emits all colors. When the atmosphere removes the blue content of sunlight, what's left is a mixture of all colors except blue. Remember, the eyes don't have any receptors for yellow. However, when all the colors except blue tickle the receptors in the eye, the colors tickle only the red and green receptors. The brain has limited information to work with, so it decides that the tickling of the red and green receptors must mean the sun is yellow.
When the yellow sunlight that reaches the earth is mixed with the blue of the sky, the colors are added together in snow to make white snow.
But then again, don't just take my word for how the eyes work, Google "color receptors in the eye", or check out these sites.
https://www.pmg.com/blog/color-analysis-when-designing-for-mobil-devices-part-1-color-theory/
http://www.brighthub.com/multimedia/publishing/articles/100762.aspx
Douglas Krantz
