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### Re^2: How many colors does a rainbow have?

by Gavin (Chancellor)
 on Jan 25, 2009 at 18:34 UTC ( #738791=note: print w/replies, xml ) Need Help??

in reply to Re: How many colors does a rainbow have?
in thread How many colors does a rainbow have?

"3" As in the others can be "synthesised" as a mixture of Red Green and Blue?

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Re^3: How many colors does a rainbow have?
by BrowserUk (Pope) on Jan 25, 2009 at 19:49 UTC
But what if you are color blind? Do you fill in the gaps or does it just become gray?

Hm. This was (is?) beyond my level of knowledge, but interesting enough to cause me to try and understand it. What follows is what I think I've learnt.

Whilst we can assign names to specific points or narrow bands of the visible light spectrum giving a one to one correspondence, human color perception is (a lot) more complicated.

In the normal (average) human eyes, there are three types of color detector, and their sensitivities overlap. And the actual color perceived is the sum of the responses from the three detector types. In addition, there is also wide variation in the numbers of these types of detectors (64% red; 32% green; 2% blue; (what about the other 2%?)), and variations in their sensitivity also. This is further complicated by the fact that the light reaching the eye (and the individual cones), is rarely pure light of one frequency, but rather a mixture of frequencies at differing strengths.

The resultant additive values (so called Tristimulus Values) do not map one to one to the spectral frequencies. Instead, there can be many combinations of the responses from the three types of cones that sum to cause us to perceive any given color.

Add to this the fact that each individuals cones may respond (be most sensitive) at slightly different frequencies, but because when we are shown a particular color we are told it is 'burgundy;, 'chartreuse' or whatever, the absolute responses our eyes detect can vary, but we perceive the same color as each other. Because that's what we have been told is the name for our particular set of responses.

The problem for color blind people is that one or more of their sets of cones has either no response, or a very low response. The affect of this is that they perceive a much wider variation of input tristimulus values as mapping to the same 'color label'. Hence, they are unable to distinguish between ranges of spectral values that most of can easily distinguish. So, for example, in red-green color blindness, the suffer has trouble distinguishing between many shades of red, yellow and green as they all tend to result in very similar tristimulus values.

This can all be further complicated by the desire or need of the viewer to make a distinction between closely related colors. Anecdotally at least, Eskimos have more words for white than the rest of us because they have a need to distinguish between them. Likewise, it is said that jungle dwellers routinely make distinctions between very similar shades of green that city dwellers find nearly impossible to tell apart, because they have that need.

Take all that with a bucket full of salt. For one thing, it is clear from the literature that our understanding of how vision combines the physical elements of the eye and the optic nerve with the more ephemeral aspects of the brains interpretations, is far from complete.

I highly recommend that anyone else that is fascinated by this to start here and reach their own conclusions.

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
Re^3: How many colors does a rainbow have?
by Lawliet (Curate) on Jan 25, 2009 at 19:21 UTC

I thought it was Red, Blue, and Yellow.

Update: I obviously do not know what I am talking about :(

And you didn't even know bears could type.

I thought it was Red, Blue, and Yellow.

That's what they teach in elementary school, but they're talking about paint, not light, when they teach you that. And it's still not quite exactly right. Technically it would be closer to magenta, cyan, and yellow, but they don't want to teach you words like "magenta" and "cyan" in kindergarten.

Each of these three basic pigments absorbs one of the major colors of light that the eye sees. (Of course, the cheap paints you work with in gradeschool don't do so perfectly, by any means, but this is how it's supposed to work. If your pigments were perfect, mixing them all together would give you black; in practice, you usually get a murky brownish color, because the pigments you have are imperfect.)

For instance, yellow pigment absorbs blue light, so what gets reflected is mostly green and red light; when the eye sees green and red, the brain interprets it as yellow. The other two pigments work similarly. For further reading on this subject, see CMYK color model.

--
We're working on a multi-year set of freely redistributable Vacation Bible School materials.
3 - Red Blue and Yellow, the other colors are achieved via dithering.

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