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Author Topic: Why do colours add up together to make other colours?  (Read 1895 times)

Offline bizerl

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It seems like a simple enough idea, but my (limited) understanding of the RGB spectrum was that it was the manifestation of different frequencies of electro-magnetic radiation.

In sound, you can add various pure-tone sine waves to make complex sound signals, but with light, you add two "pure-tone" sine waves of electro-magnetic radiation and you get...

ANOTHER PURE-TONE SINE WAVE OF ELECTRO-MAGNETIC RADIATION!! (i think).

Is this a trick of the light as it is picked up by our cones and/or rods?


 

Offline grizelda

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Re: Why do colours add up together to make other colours?
« Reply #1 on: 10/09/2012 05:40:52 »
Probably has to do with the fact that sound travels through a medium, so additional frequencies re-modulate a medium which is already being modulated. Light doesn't interact with a medium, so additional frequencies are mathematically added to the original frequency. No change is made to the shape of the waveform because it doesn't have one.
 

Offline damocles

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Re: Why do colours add up together to make other colours?
« Reply #2 on: 10/09/2012 05:51:17 »
It is a trick -- or rather a mode of reaction -- of the cones in our visual system.
The yellow that we observe from a sodium emission spectrum is quite different from the yellow that we perceive indistinguishably in transmission from a sodium chromate solution. The former comes from a wave that is the sum of just two wavelengths of yellow light (possibly made slightly orange by admixture of a weaker red emission line). The latter is a mixture of all wavelengths of green, yellow, and orange light that are the only visible light that is not absorbed by the solution as white light passes through it.

Irrelevant, but chemically it is possible to exactly match the subjective colours of the chromate solution and sodium lamp in a particular situation by lowering the pH to increase the amount of orange dichromate in the yellow chromate solution.

The cones in our eyes are tuned to respond to three separate peak wavelengths, with a sensitivity spread that looks roughly like a set of three overlapping gaussian peaks. The same set of three numbers given by the strength of the eye's response to the three key frequency ranges might therefore represent either a single frequency or a broad frequency spread. Of course, for some particular colours -- brown, pink, magenta -- the signal received by our eye could not correspond to any single frequency.
 

Offline CliffordK

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Re: Why do colours add up together to make other colours?
« Reply #3 on: 10/09/2012 07:36:37 »
Keep in mind that there are two different ways to "mix colors".

When mixing paints, one may absorb more wavelengths of light, and reflect less.  So, mixing all paints together, and one ends up with black (all light absorbed, none reflected)
Mixing lights, on the other hand, one adds more and more wavelengths of light, and ends up with white.

The colors one gets from mixing paints and mixing lights are different.

The French Impressionists were experimenting with mixing light by combining the dots of pure colors.

I believe that color laser printers also print dots of pure color, whereas the color inkjets may mix the colors somewhat.  The colors on your computer screen are also made up of 3 colors, Red, Green, and Blue, of varying intensity.

As Damocles put it.  One has 3 cones responding to varying overlapping colors spectra.

Pure yellow would cause a probability of each cone being activated, which the eye/brain then interprets as yellow. 
If one uses mixed colors of multiple wavelengths that give the same exact probability that the three cones are activated, then the eye should interpret it as yellow.
 

Offline evan_au

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Re: Why do colours add up together to make other colours?
« Reply #4 on: 10/09/2012 12:44:07 »
The three cones of human vision have a frequency response shown here: http://en.wikipedia.org/wiki/Color_vision#Physiology_of_color_perception
The rainbow is made up of pure colours, which generally group into 6 or 7 names (depending on what you were taught when you started school).

The hairs of the cochlea respond to many different frequencies, and can distinguish the 88 notes on a piano keyboard (plus more) - although not many of us could name a specific note we heard! http://en.wikipedia.org/wiki/Cochlea#Summary

You could display a mixture of Red and Green colour, which will strongly stimulate the both the Medium and Long-wavelength sensitive cones. A pure Yellow frequency from the rainbow will also stimulate the same Medium and Long-wavelength sensitive cones, and so be perceived as the same colour.

But this does not imply that your eyes are performing a non-linear mixing of two light wavelengths to come up with a third intermediate wavelength. It is just that the two wavelengths stimulate the same cones at the same strength as a single, intermediate wavelength.

A counter-example to this theory is if you mix Red and Blue light, you end up with Magenta, which is not one of the pure colours between Red and Blue. See the colour circle at:
http://en.wikipedia.org/wiki/Primary_colour#Additive_primaries
 

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Re: Why do colours add up together to make other colours?
« Reply #4 on: 10/09/2012 12:44:07 »

 

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