What is colour?

Squashie

Is colour an intrinsic property of the materials around me, or is it merely an effect created by my brain in response to different wavelengths of electromagnetic radiation incident upon my optic sensory nerves?

(I should be cramming for an exam in 19 hours but my line of thought has kind of trailed in the past hour after this thought randomly popped into my head.)

Two years of physics tell me that that actually is true. Everything around us is essentially colourless and dark, however emitting or reflecting wavelengths of EM radiation, which luckily our eyes have just adapted to sense? Am I right? Am I wrong? If I'm wrong there must be some sort of quantum theory of colour?... or something?

deckstunt

There was a recent program on BBC with marcus du sautoy (i think) and as part of it he answered that question and his answer was colour is all in the mind, it doesn't exist in the 'real' world.

The program had a good experiment to illustate this. It was two computer images of something like a Rubik's Cube. In both there were certain coloured squares that were the exact same colour, say blue. In both images all the other squares were white. But as the experiment went on the the white squares were colored in differently in each image

In one the blue squares now 'looked' say yellow. And in the other the blue squares looked green. It was very surprising.

deckstunt

Actually it was an episode of Horizon. And you can see the experiment on You Tube:

http://www.youtube.com/watch?v=KDNsgBbVg2c&feature=player_detailpage#t=336s

Sev

Correct me if I'm wrong, but I believe your brain's sensation of colour is based on how much you stimulate the three light sensitive cells in your fovea which pick up on a range of frequencies centered with some spread around red, green and blue.

A sodium vapour lamp, I believe, produces a monochromatic signal of exactly one particular frequency. This is a very pure frequency of 589nm. However, there is no "yellow sensor" in our eye, specifically designed to capture light of 589nm, but... 589nm falls within the catchment area such that it stimulates the red-centred receptors with, say 60% strength and stimulates the green-centred receptors at say 40% strength. But 589nm is so far from the frequency at which the blue receptor is most sensitive that it doesn't stimulate these cells at all. The brain interprets this result as yellow.

I believe you could have any number of objects, that emit light with any variety of spectrum shapes that will have the same result. As long as the incident light stimulates the red receptors with 60% strength, and the green receptors with 40% strength, then it will appear as the same colour, even though it could actually emit radiation with a very different spectrum.

For example, you could produce the same effect of yellow in the brain, if the incident light signal is made of 60% pure "red" light and 40% pure "green" light. (More specifically, these would be the relative magnitudes of the fourier components of those frequencies in the EM signal).

To put it another way. You could choose to define colour as the exact radiation spectrum of a particular radiation emitting object. But as far as the human eye is concerned, there will be a degeneracy. Many objects with similar but different radiation spectra will be interpreted as the same colour.

I could be wrong about this interpretation, however. It's what I believe to be the case but I'm not an eye-physicist

deckstunt

What I get from the Horizon program 'colour perception' is entirely a phenomenon of the brain. Regardless of the wave length of light being seen (and as a consequence regardless of retina receptors that are stimulated).

Take a look at the experiment in the link above.

Sev

Sure, I get that. But that doesn't necessarily mean what I have said above is false. I would consider the illusion that you are referring to in the Horizon documentary as a separate higher level brain function.

For example, this is the exact same illusion here:

http://www.michaelbach.de/ot/lum_adelsonCheckShadow/index.html

It's just with grey alone, which you could argue is not a "colour". Of course grey is really just what the eye interprets as a "white noise" EM signal.

The interesting fact that I'm trying to communicate is the fact that if you compose an electromagnetic signal by the superposition of the three frequencies representing Red, green and blue in the right proportions, then your eye will interpret that as devoid of colour.

An electromagnetic signal that is simply Gaussian "white noise", will also be interpreted by the eye as white because it will also stimulate the three colour receptor cells with equal intensity.

However at a higher level again. The brain's interpretation of these colours in the context may vary again. As shown in the horizon documentary.

Again, I could be wrong, but this is my understanding of it.

Squashie

It really is amazing to think then that everything we look at is actually devoid of colour. It's all an illusion created by our brains. I'll have to watch that full program some time soon.

Deckstunt wrote:

/Colour perception/ Regardless of the wave length of light being seen (and as a consequence regardless of retina receptors that are stimulated).

I'm not sure if this is what that part of the program was saying though. If I shine a monochromatic light into your eyes – a light of one wavelength only, you'll perceive a colour. Another monochromatic source of differing wavelength shined into your eyes will be perceived differently. Would it not make more sense to believe that the brain may be able to "weigh up" a wavelength signal from the focal receptors and divide it into relative ratios of an illusory "red", "green" and "blue"? In your conclusion you seem to neglect what is actually real and what isn't. EM radiation is what is real, and this radiation is classified according to different wavelengths. In order for a human (or any organism) to sense its surroundings, it must evolve a system which can detect what is real. In the case of "vision" however, differentiation between various types of EM radiation would also be a necessity. In fact "vision" wouldn't be "vision" if we couldn't perform this differentiation. Therefore I imagine this illusion that is colour is entirely dependant on the wavelengths of incoming radiation.

ride-the-spiral

It's also true that Pink (Magenta) isn't a colour as it does not actually have a wavelength of light associated with it but is created by our brain as the complementary colour of green.

thecatspjs

ride-the-spiral wrote: »

It's also true that Pink (Magenta) isn't a colour as it does not actually have a wavelength of light associated with it but is created by our brain as the complementary colour of green.

cool

himnextdoor

I think that colour is a result of evolution. Being able to distinguish green grass from sand enabled us to deduce where we might find meat; being able to recognise blue gave us a visual cue for water (reflecting sky) and red enables us to know the difference between blood and water.

It is not surprising then that our eyes and brain have evolved to make use of this crucial data. In many other animals, the sense of smell has evolved to interpret the same source of that data and have the advantage of not being thwarted for a lack of light.

So, through natural selection, the 'cones' (colour sensitive cells in the eye) have evolved to respond strongly to red, green or blue stimuli and to respond weakly to EM corresponding to other colours in the spectrum.

When the eye looks upon say gold, for example, the red and green receptors are being weakly stimulated and we call this yellow. Yellow is a 'learned' colour and is generated by the mind on the basis that gold is valuable. Yellow is a result of 'social evolution', and could be regarded as an 'archetype'.

Gold is of no use to lions or zebras so they do not see it. Yellow is a waste of energy to bears and bison so they don't bother with it.

franb7111

is the colour yellow i see the same "colour" yellow that you "see"? or just my brains interpetation of the colour yellow. maybe no two people see the same colour yelow. its posible that what i see as a yellow flower somebody else sees red but because you have been told at an early age that that particular colour is yellow, everthing red you see you call yellow!!!!

SkepticOne

I think there is an objective aspect to colour that can be defended without contradiction. I think the reason we tend not to recognise it is that our sense of colour is very crude compared to the objective physical phenomenon the sense is designed to detect.

Is sound the sensation we get when we hear something or is it mechanical vibrations in air? This is the question underlying the riddle about the tree in the forest.

There are two aspect to the the concept of sound, the subjective and the objective. The objective aspect is the mechanical vibrations in the medium.

The equivalent objective aspect to the concept of colour is the spectrum of light.

(Note: the objective aspect of colour is not a frequency or wavelength but the spectrum in the visible wavelengths. Pink is a colour in this sense even though it may not be a single wavelength. )

Our colour vision is an attempt to glean information about the spectrum either of direct light or more importantly the spectrum of reflected light off an object.

Our means of sensing the spectrum is very crude however. We only have three types of colour detector that respond to three broad overlapping regions of the spectrum. Two objects could have very different spectra but provided they produce the same level of stimulation in the three detectors, they will be perceived as having the same colour.

Where the sophistication comes in is at the level of the brain. Because we have to determine the colour objects under vastly different illumination we have evolved the means of correcting for illumination based on the overall colour of the surrounding scene. We don't merely measure the colour of the object itself. This mechanism is responsible for some of the illusions in the TV programme mentioned earlier.

FISMA

You have to have a look at this applet - you shouldn't believe your eyes!

http://www.michaelbach.de/ot/mot_mib/index.html

Basically, focus on the green dot. In your peripheral vision, the yellow dots will disappear.

Actually, when you focus on the yellow dots, you will determine that they never disappear! It only appears that way!

Your brain lies!:D