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/

[xoxoxoBruce]

Ah ha! So that's why when I mixed all the jars of poster paint together, I got brown instead of white......or even black. Also a ration of crap from Mrs Midyet.
Superb explanation, linknoid. That explains a lot of things that didn't quite jibe with what I'd been taught. Thanks for taking the time to expand on it so we could all understand it.
YOU, yes you, lurking out there. You didn't know that either, did you? You were waiting for me to make a fool of myself so you could find out the straight skinny. That's OK, I do it all for you. Happy New Year.

btw- Great link...I'm saving that one.

[BigV]

XOB:

This picture shows an example of the additive color mixing. The most striking example of this I ever saw was in the Hard Rock Casino in Las Vegas. There was a white wall with some cast metal letters mounted on it and the letters all had shadows of different colors. It just about drove me crazy. My poor brain was trying to reconcile the proper shape and angle and area for all the shadows with the obvious problem that they were not "dark". This image recreates the effect as well as any simple 2d picture can.

Here is the link for the science behind the image.

http://www.newtrier.k12.il.us/academ...t/coloshad.htm

EDIT: I found a picture and an explanation I like better.

http://www.exo.net/~pauld/summer_ins...ploration.html

[capnhowdy]

as an artist , I'm compelled to post:
you cannot acheive, (w/ paint & canvas) what I'm reading here. One cannot confuse the spectrum of color with what is created with physical color mixture. The color spectrum is a basic guideline to the physics of color, which is scientifically the explanation to "everything that shallow people will relate to". There are no limits to color and the effects on the mind. Which is to say.... what you may see is not what someone else will see. I've tried some mixtures as suggested in this thread, and if You'll do the same you'll realize that it is totally hypothetic and "textbookish". Color, let alone art, is what is absorbed in one's mind & their imagination. What looks green to me may be teal to you. What is provocative to you may be calming or docile to me. In the minds eye, of course. There is no scietific proof that we even see color at the same level or hue or tone, etc. Especially intensity. Color is a very personal thing. " I saw red" may mean to you that you were really pissed while to someone else may mean they were totally elated.

[linknoid]

Originally Posted by capnhowdy

as an artist , I'm compelled to post:
you cannot acheive, (w/ paint & canvas) what I'm reading here. One cannot confuse the spectrum of color with what is created with physical color mixture. The color spectrum is a basic guideline to the physics of color, which is scientifically the explanation to "everything that shallow people will relate to". There are no limits to color and the effects on the mind. Which is to say.... what you may see is not what someone else will see. I've tried some mixtures as suggested in this thread, and if You'll do the same you'll realize that it is totally hypothetic and "textbookish". Color, let alone art, is what is absorbed in one's mind & their imagination. What looks green to me may be teal to you. What is provocative to you may be calming or docile to me. In the minds eye, of course. There is no scietific proof that we even see color at the same level or hue or tone, etc. Especially intensity. Color is a very personal thing. " I saw red" may mean to you that you were really pissed while to someone else may mean they were totally elated.

As a physicist (well not really, but I did major in it in college for a while), I would say that you're correct (at least in this comment, the next one where you say to throw out the spectrum completely is going overboard). In my explanation I specifically avoided going into the complexities of perception or even the details of what happens when you mix color. The point was to explain the basics of how colors work to someone who claimed not to know hardly anything about it.

Each person's eyes respond to slightly different wavelengths. In fact, some men only see 2 shades (they're colorblind), and some women actually, because of genetic issues, have 4 types of cones instead of 3, and they actually see a much different colors (and they're genetics mean if they pass on the genes that produce 4 colors for them, their sons will be colorblind).

But once you get past the issue of how each person responds to the various (and infinite) combinations of the colors of the spectrum, then you have to deal with where that combination of wavelengths and intensities are coming from.

First you have the light source. Each different source is different. The sun produces a relatively complete visible spectrum, resulting in very white light, but by the time it's passed through the atmosphere, a lot of the blue end has been scattered out, and even then the spectrum changes based on the time of day, the weather conditions, the pollution in the air, etc. And there are many other different light sources: incandescent lights, LEDs, mercury vapor lamps, halogen lamps, candles, wood fire, arc lamps, flourescent lamps, and the list goes on. None of them really produce a pure, even spectrum, and each one of the wavelengths it puts out interacts differently with different materials.

Then once it's produced by the light source, it has to deal with absorbsion (and re-emission as other colors), reflection, refraction, transmission, scattering, interference, and who knows what else. So when you mix two paints, you have to account for all those other things if you really want to know what color you're going to end up with. The thing the printing industry tries to do is simplify everything enough that they can reproduce most colors based on just a few, for practical reasons. Which is why we use primary colors. But that's definitely not the whole story.

So I hope you'll forgive me for make the vast simplifications to make the mixing of colors understandable and not overwhelming.

[capnhowdy]

Originally Posted by linknoid

As a physicist (well not really, but I did major in it in college for a while), I would say that you're correct (at least in this comment, the next one where you say to throw out the spectrum completely is going overboard). In my explanation I specifically avoided going into the complexities of perception or even the details of what happens when you mix color. The point was to explain the basics of how colors work to someone who claimed not to know hardly anything about it.

Each person's eyes respond to slightly different wavelengths. In fact, some men only see 2 shades (they're colorblind), and some women actually, because of genetic issues, have 4 types of cones instead of 3, and they actually see a much different colors (and they're genetics mean if they pass on the genes that produce 4 colors for them, their sons will be colorblind).

But once you get past the issue of how each person responds to the various (and infinite) combinations of the colors of the spectrum, then you have to deal with where that combination of wavelengths and intensities are coming from.

First you have the light source. Each different source is different. The sun produces a relatively complete visible spectrum, resulting in very white light, but by the time it's passed through the atmosphere, a lot of the blue end has been scattered out, and even then the spectrum changes based on the time of day, the weather conditions, the pollution in the air, etc. And there are many other different light sources: incandescent lights, LEDs, mercury vapor lamps, halogen lamps, candles, wood fire, arc lamps, flourescent lamps, and the list goes on. None of them really produce a pure, even spectrum, and each one of the wavelengths it puts out interacts differently with different materials.

Then once it's produced by the light source, it has to deal with absorbsion (and re-emission as other colors), reflection, refraction, transmission, scattering, interference, and who knows what else. So when you mix two paints, you have to account for all those other things if you really want to know what color you're going to end up with. The thing the printing industry tries to do is simplify everything enough that they can reproduce most colors based on just a few, for practical reasons. Which is why we use primary colors. But that's definitely not the whole story.

So I hope you'll forgive me for make the vast simplifications to make the mixing of colors understandable and not overwhelming.

Thanks for the input, Link...... highly informative. I must apologize as frequently my creativity and preference to the abstract bullheadedly ignore the facts and physics of almost any subject. You know what they say about us...ahem...artists. I'm still not sure if I have a talent or a curse. While I live in a world of imaginative creativity on a daily basis, I find it difficult at times to mindframe "the real deal". Thanks for the reality check. Please bear with me.........

[cweekly]

thanks for the clarifcation/correction, linknoid
(sorry xobruce, didn't mean to lead you astray)

so I do think I was right about the striations, but yeah was off base wrt the doppler effect's relevance to the colors. didn't quite jibe with me either, glad to get to the root of it.

"crippled but free, I was blind all the time I was learning to see..."