Say I’m looking at a color like green. Is there a way I can tell if it is pure green (i.e. light at a single wavelength that coresponds to green) or if it really is yellow and blue mixed together and just looks green? Can I tell the difference just by looking or do I need some sort of optical device like a prism?
You could possibly do it with narrow band filters, but a prism would be better.
Not to get philosophical or anything, but I fail to see the difference between “being pure green” and “just looking green.” A color is defined by how it looks.
I think if a graphic artist is around here somewhere (s)he might tell you that there is no “pure green”.
I think all the colors (in printing anyway) are made up of varying mixtures of Cyan, Magenta, Yellow and Black.
I’ll just sit back quietly and wait to be proven wrong by a scientist or something now… 
There is a distinct difference to physicists; it is possible to mix red (~650nm) and green(~540nm) light to produce a beam that the human eye will perceive as yellow, and it may look exactly like a beam of pure yellow(~600nm), but it remains a mixture and can be separated back out to the original components - a beam of pure yellow cannot be split.
i’m no scientist, but i do believe that the difference between additive (print… CMYK), and subtractive (light, RGB) needs to be pointed out.
i know there are other (and better) models out there, but i’m an idiot.
This is the situation I was wondering about. I was at a science museum once where they had three colored spotlights focused on the wall in such a way that they made intersecting circles. Where the spotlights overlapped, I would see a different color. I know that the different color I’m seeing is not light at a single wavelength–rather it’s two separate wavelengths that are fooling my eye into perceiving it as one color. I was wondering if there was a way I could visually tell between false yellow made up of two wavelengths versus true yellow at a single wavelength.
I think it is true to say that not everyone perceives the ‘false’(composite) colours in the same way - meaning that if you had a pure yellow source and two variable sources (one red, one green), then faded them so that the composite colour appeared to match the pure yellow, someone else looking on might not see them as exactly matching and would adjust the faders differently to achieve what they consider to be an exact match (whereupon you would see it as an inexact match).
I think it may even be true that it is possible for a person to see the composite colours differently with either eye.
I’d like to offer a conjecture to anyone who wants to set up an experiment.
Apparatus:
A Yellow LED
A Bicolour LED (of the type that contains one red and one green chip, having three leads so that both chips can be powered simultaneously)
Potentiometers etc.
A sheet of bright red paper
A sheet of bright green paper
Set the yellow LED and the bicolour one up next to each other so that the potentionmeters can be used to vary the intensity of red and green light in the bicolour LED.
Adjust the potentiometers so that the two LEDs are indistinguishable in colour.
Stare closely at the red paper for a minute, then look at the two LEDs - observe any differences in apparent colour.
Rest, then repeat, staring at the green paper.
I think that it will be thus possible to distinguish between composite and pure colours.
There might be a way when u are dealing with a light source, and not paint.
Get several slips of construction paper, each being a different color of the visible spectrum… (R O Y G B I V) , plus a few shades of grey just in case.
Put them under the light… If they do or don’t change color, (which, i forgot) then it’s pure.
EX: Yellow light… put yellow paper under it… if it’s pure yellow, the paper will “seem” white, or no change. Now put a green paper… if it still looks green, then the yellow light has green waves in it.
Now, this was back in 7th grade, so I may have this totally off. IANA Scientist.
Just using your eyes, no. It’s to do with rods (or is it the cones?) in the retina and the way you perceive colour. It’s not fooling the eye. That is the way eyes work. For instance you can’t have ‘pure’ white light becasue any ‘white’ light must be a mixture of wavelengths.
You need a prism. If you shine a laser* beam (lasers shine at a single wavelength) though a prism it just bends - it does not split, if you shine normal light through the different wavelengths get bent by different amounts and you’ll get a spectrum of some sort.
*there are some devices that lase at more than on frequency, but we’ll ignore them.
Red Yellow Blue. . .primary colors. They are not made up of any other color.
All other colors are combinations of those three. . .secondary colors.
Pure Green may mean something in a particular discipline, but it is still a combination of Yellow and Blue. That’s how green is produced.
Black and White. . .absence of all color and all colors mixed together.
hroeder: No, you’re just wrong here. Re-read Mangetout’s post. Anyway, it’s Red, Green, Blue.
Ok, something I’ve always wondered about: why is it that in elementary school we learn that red, yellow, and blue are primary colors, when in fact they are neither primary colors of pigment or light? Still, yellow and blue DO make green when you’re talking grade school art projects. Is this because the red is similar enough to magenta, and the blue similar enough to cyan, to make this work - and are colors mixed from red, yellow, and blue muddier as a result?
I always wondered about this myself since learning how color reproduction works, long long after grade school was a dim memory. Glad to know I’m not alone.
My first guess was that I’ve misremembered the colors: the blue and red paints were really cyan and magenta, but the teachers lied and called them “blue” and “red” to avoid all the irritating complications. (Likewise: “Oh by the way, the Civil War was all about slavery. See you next fall!”)
But gosh, it really seems like those paints of childhood were a true red and blue. Could they have been? That would mean my memory is accurate, my teachers were telling the truth, and that the laws of optical physics have changed since my youth. For a brief, happy time, this solution satisfied me.
On further reflection, I started seeking an even more satisfying explanation, ideally one that would salvage physics as we understand it.
My final theory then is much like yours: the paints really were red and blue — or at least closer to red and blue than the proper magenta and cyan inks used in printing — and therefore their mixtures were impure but good enough for the kiddies not to notice the difference, or care. That is, blue and yellow grade school paints mix to produce green, but it’s a darker off-color green compared to the “true” green you get from yellow and cyan.
Ideally, a professional who knows what he’s talking about will come by and explain this properly.
If you’re asking how you could do this from the audience with available materials: a prism might not be at hand, but a diffraction grating (common in novelties and some optics). a hologram on a credit card, or it you are really skillful, create an infinitesmal slit between two edges. If you wear standard bifocals, or have an SLR camera with split right focusing, you may be able to orient them to generate a fringe rainbow at the interface between lenses.
The primaries for light are red, green, blue and the primaries for ink are magenta, cyan, yellow because ink works by absorbing (subtracting) light of certain wavelengths, whereas mixing light beams involves adding them together.
In primary schools though, red and blue are substituted for magenta and cyan - I’m not sure of the exact reason for this (it may be that pure red and blue pigments are easier/cheaper to make than magenta or cyan, or it might just be that it is good enough - the kids only mix all three together to make a sort of muddy purplish brown anyway, so why bother being too precise?
I should quickly add that these systems of primaries aren’t perfect in themselves, just conventional.
Pigment colors and light colors behave differently. The primary pigments are RGY, while the primary lights are RGB.
I was recently given a load of open bulk packs of powder pigments from a primary school (they just buy paints ready-mixed now) - the red is (my descriptions - the labels just state the colour name) pure crimson, the yellow is bright chrome yellow and the blue is a deep blue that does seem to tend towards the turqouise/cyan. Also included were some beautiful deep purples, gold, silver and a couple of packs of ‘pink’ which is to all intents and purposes, magenta.
Like you, I’m sure we mixed up red, yellow and blue at school, but I’m also pretty sure that we had non-primary pigments available too.
Wuh duh fuh?
Red, green and yellow?
How would make blue from these?