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From: dfc@hpindda.HP.COM (Don Coolidge)
Newsgroups: sci.misc
Subject: Re: Color
Message-ID: <3290001@hpindda.HP.COM>
Date: Fri, 4-Dec-87 18:31:42 EST
Article-I.D.: hpindda.3290001
Posted: Fri Dec  4 18:31:42 1987
Date-Received: Thu, 10-Dec-87 19:38:06 EST
References: <162300002@uiucdcsb>
Organization: Hewlett Packard, Cupertino
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> I've got two miscellaneous science questions. 

>1) On TV's and computers screens, why is it RGB (red, green, blue)
>instead of RYB (red, yellow, blue) the primary colors?

>2) Some light wave length produces the color green. A mixture of
>the wave lengths of blue and yellow also produces green.
>Even though these two greens are indistinguishable to our eyes, are there
>(could there be) instruments that distinguish them?
----------

Your two questions are actually one...red, blue, and yellow are the primary
PIGMENTS, but red, blue, and green are the primary LIGHT COLORS. Red pigment
absorbs blue and green light; yellow absorbs blue and red; blue absorbs red
and green. Mix blue and yellow pigment and everything is absorbed except
green wavelengths, which are reflected. 

Green light does not come about from mixing blue and yellow light, though; 
its wavelengths are intermediate in frequency between blue and yellow. Shine 
a white light through a blue filter and a red filter in sequence, and only 
the green light will make it through. (No, I'm not talking about a blue- or 
red-COLORED filter - I'm talking about one that filters out blue or red light.)

So, in pigments, blue plus yellow makes green. In filters, white minus blue 
minus red makes green.


Don Coolidge