The meeting of blue and red on the color wheel has always
bothered me because it is illogical in terms of light frequencies. The
relationships described by the color wheel are helpful but I don’t think people
think about color enough.
The sky is blue because of Rayleigh scattering. But that is
the type of physics “instruction” I hate – just give a name to a phenomenon and
move on. What does that do for me? Now I have a name, I still don’t understand
what is going on. Descriptive not explanatory - very unsatisfying.
Sunrises and sunsets provide beautiful colors because of the
distinctive colored photons produces by the Sun. The Sun produces tons of photons
at distinctive frequencies like in a rainbow.
We call them colors if they fall in the frequencies to which our eyes
happen to be sensitive. White light is produced by the mixture of these colors,
the Sun is not generating white light. Although from space it looks white, just
like any other star, because we mentally recombine the distinctive colors.
Why does the Sun appear yellow to us? It appears this way
because the shorter frequency (blue) light gets scattered more in the
atmosphere than lower frequency colors. The blue is subtracted from the light
produced by the Sun as it makes its way to your eyes. Because it is missing
some blue – stolen to make the blue sky—the Sun appears yellow.
The physiology of our eye is such that we see a range of frequencies
with a peak in the middle as the typically three types of photoreceptors have
peak sensitivity above at and below the green frequency. A tiny percentage of
people are tetrachromats and have four photoreceptors. There view of the world
is so rich that they don’t have language to describe it. A tiny percentage of
the people have various types of colorblindness and they too see the world
differently than most.
The blueness of the sky changes with how much atmosphere you
are looking through, that is why it is darkest straight overhead. The
atmosphere is a blue filter. When painting a sky, you lay down white paint at
the horizon and blue at the top of the canvas and blend them together.
Our eyes are picking out frequencies and our brain combines
them so we don’t see reddish green but we do see yellow. What we see is illuminated
by incoming light, from the Sun, the sky, a lamp and reflections from all sorts
of surface. This makes the study of color and its portrayal in art very
interesting. And humbling. Our physiology is different so we might be seeing different
things. When Monet went through his “blue
period”, he may have been less able to see blue and therefore intensified that
hue. How do we use words to explain the colors we see? Why is the sky darker
outside a rainbow than inside? What I see in my painting might not be what you
see.
The color pictures from Mars Rover are a spectacular reminder that the sky is not blue on Mars. Instead, it has colors that have been described as everything from "orange-pink" to "gray-tan", as was discovered in the 1970s by the Viking landers. This is because the atmosphere of Mars is very thin and dusty, and atmospheric light scattering is dominated not by the molecules of gas (in the case of Mars, mostly carbon dioxide) but by suspended dust particles. These are larger than the wavelengths of visible light, and they are reddened by iron oxide, like Martian soil. It's not just Rayleigh scattering, so the power spectrum is different.
