12/30/2004: Green flash at sunset

[linknoid]

Originally Posted by xoxoxoBruce

If I remember art classes correctly green is yellow & blue and violet is blue & red.
So how can we get green if the blue component in already dispersed?
And how can the violet get dispersed if half of it is red yet the red doesn't?

The short answer is that there are two ways of mixing colors. One is by mixing what is absorbed, subtractive mixing, which happens when you mix paints, and the other when you mix colors of light, additive mixing. The mix of colors coming from the sun is the dividing up of white light into the colors of the rainbow, not of mixing pigments.

Our eyes only see 3 colors: red, green, and blue. Using those 3 colors you can simulate most any color, which is why TVs and monitors only use red, green, and blue (RGB). There are 3 types of cone cells in your eyes, one for each of those colors, so every every color you see is based on the proportions that each of those cells are activated.

When you see yellow, it means that both red and and green receptors in your eyes have been activated. Yellow light activates both red and green receptors (since its wavelength falls in between red and green), but if you mix red and green light, it has the same effect of activating red and green receptors. And when you see either yellow light or red and green lights mixed, you can't tell the difference, your brain just interprets it as yellow.


On the absorbtion side of things, the primary colors are cyan (anti-red), magenta (anti-green), and yellow (anti-blue). That's why color printing uses Cyan, Magenta, Yellow and blacK (CMYK) instead of red, green, and blue to reproduce all the colors. Cyan absorbs Red light (and reflects green and blue), magenta absorbs green, and yellow absorbs blue, so they're the exact opposite of the 3 primary colors of light. If you take a picture on film, the negative will turn reds into cyans, magentas into greens, and blues into yellows.

But when you mix paints, you aren't mixing pure red, pure yellow, and pure blue. It's hard to tell what light frequencies are being absorbed without using a spectroscope, so the exact colors you see when you mix two colors of paint all depends on which frequencies (colors) of light are being reflected and which cells in your eyes are being triggered by each frequency, and how strongly.

It's not a simple subject at all, and I'm not going to attempt to explain any more here. Hopefully everything I've written here is understandable. If you want to know more, there's an excellent (and much more technical) explanation here:

http://hypertextbook.com/physics/waves/color/