is spin mad or correct

Spin has a couple of points here.

1) You really do only see light in three colour bands. 564-580nm, 534-545nm, 420-440nm.

Yellow light is 570-590nm and is detected by the interaction of your RED or LONG cones, minor stimulation of your GREEN or MEDIUM cones and the amount of light on your RODS.

Turquoise on the other hand just barely stimulates your GREEN medium and BLUE short CONES, (and a tiny amount of RED long) cones. So Turquoise is what your brain invents when it's getting a lot of light with a little blue and a little green in it.

But to answer the original question, Spin is either mad or a troll. I hope for his sake he's a troll.

There are 2 ways we perceive yellow. Light with a wavelength of around 580nm appears yellow to most humans. This is the yellow in the spectrum (red, orange, yellow, green, blue, indigo and violet - an unreliable source (QI) tells me there are 7 colours in the spectrum because Newton beleived that 7 is a luckier number than 6) produced by splitting white light or sunlight.

Alternatively we can produce the appearance of yellow light by mixing green and red light - this stimulates the same 2 sets of light sensitive cones in our eyes as 580nm or proper yellow light so appears identical (if you get the ratio of red to green correct). Unless you're colourblind in which case you (quite correctly) see that they are not the same.

So there is such a colour as yellow, but there's also a false/impostor yellow made from red and green!

Spin is incorrect twice:
1. There is yellow, it is light with a wavelength of around 580nm
2. Blue and green light mix to produce Cyan. Yellow light is a mixture of red and green.

Hi Spin

As an amusing sideline, qualia , like the perception of colour is seen to be an argument against physicalism. According to physicalism, only small, colourless tasteless bits of matter exist. So the brain and all its processes must also be only colourless bits of matter.

So how come colour? Physicalism must be false.

My understanding at this point based on arguments made from psychological testing, is that colour is something we do in the brain. The test I am aware of is the change of colour in a light that occurs when one looks quickly away from it. It goes from yellow to redish as I recall.

That said, I am also reading positions arguing against this and for realism in such matters.

Yellow exists as a physically -- and near universally -- discernible phenomenon. It exists as the product of a specific external stimulus on our biology. It is no more -- but no less -- real than any other other sensation we perceive. Yellow is the means by which we can recognise specific activities of light: It is our means of recognising light having an approximate wavelength of 580nm and, coincidentally, of recognising the equal additive mixing of 'red' and 'green' light primaries (ie white light sans 'blue' primary), which produces the same visual effect via our sensory capabilities. It's true,therefore, that 'yellowness' exists *as a sensory perception*, but *it exists* and is commonly recognised by all normally sighted human beings (ie they can all broadly agree on a concept of 'yellow'). Whether the 'yellow' any single person perceives 'looks' the same as the 'yellow' another perceives is impossible to determine (many of us, as children, will have wondered whether the colours others see actually match the colours we see).

It could probably be argued that there are/are not messages on this screen. We perceive meaning in unique arrangements of 26 symbolic characters. Perception is possible through interpretation of sensory data. Letters themselves are merely small areas contrasting with their background to generally consistent two-dimensional co-ordinates (in this case generated on screen by specific electronic impulses). Never-the-less, you could all read this post. It is -- in all meaningful respects -- real.

Your eyes perceive the wavelength they are attuned to. What you "see" is not representative of universal visual perception. There is no such thing in nature as "yellow". The "yellow" you see is a mixture of wavelengths, not actual colour. But hey, who cares, right? I am mad.

Most of the objects you see and perceive as yellow are as you quite correctly state a mixture of wavelengths which happen to stimulate both our long and medium wave length cones, giving us the impression of yellow

However, you are wrong to say this means that yellow doesn't exist. It does; yellow light is electromagnetic radiation with a wavelength of around 570-580nm. Due to the nature of human colour vision we are unable to distinguish this single wavelength from a mixture of wavelengths so many of the things we perceive as yellow perhaps aren't - they are a mixture of other colours/wavelengths which our eyes cannot separate.

If you want to see yellow, try buying a 580nm monochromatic filter and placing it in front of a light source. it will only allow light with a wavelength of 580nm to pass through it and therefore you now have a source of yellow light (most yellow filters just block a little of the blue light leaving all the other colours - white light with a little blue removed will appear yellow. But you really want a filter that blocks all the red, orange, green and blue light).

Alternatively use a glass prism to split a source of white light (from an incandescent bulb or the sun) onto a white surface. The bit between the orange and the green appears yellow because it is. Any colours which you can't see in the spectrum produced (such as brown) you could argue don't exist - they're just mixtures of other colours. But yellow was a bad choice - it exists.

Interestingly the screen you are reading this on cannot display true yellow - it can only make a mixture of red and green (have a look at the pixels through a magnifying lens), but it's good enough to fool our eyes.

I recently saw a demonstration in which 2 apparently identical sources of orange light were shone onto different objects (the actual light sources were hidden from the audience). When shone onto a white surface the 2 lights appeared identical to most of the audience, except those who were red-green colour-blind who could tell that the 2 sources were different. One source was an orange LED, the other was made of 2 LEDs; one red and one green. An orange object appeared black under the red-green mixture as it only reflected true orange light (around 600nm) which neither LED emitted.

A similar demonstration could be done with yellow, but it's not easy to find an LED which emits yellow light - so yellow is rare, but it isn't none-existent!

IN: "Yellow" exists only in the eyes of humans. "yellow" is the light bounced of objects that cannot absorb the wavelengths of what we call "Green" and "violet". "Yellow" to the eye of bee is "purple". Therev are no colours. What you see is reflected light gfrom things. An apple is every colour EXCEPT green; green is the colour it rejects...

"reflects" does not mean the same as rejects.

There are 2 kinds of colours - those in the spectrum where each is made of a single frequency and (more commonly) those which appear similar but are in fact made of a mixture of wavelengths. You are semantically obsessing over the latter whilst ignoring the existence of the former.

You are confusing "bee-purple" which is a mix of UV and yellow with yellow (human eyes can't detect UV so both would look identically yellow to us). Bees' eyes are apparently able to distinguish a mixture of wavelengths from a single wavelength - something we can't do. So they would be able to distinguish single frequency yellow from a mixture which we would perceive to be the same.

IN: Every colour you see is a wavelength reflected from the object. As For "bee-purple" I suggest you submit that theory to your local scientific department. Light is light, regardless of who or what perceives it.

Spin:

You said: ""Yellow" to the eye of bee is "purple"."
and then you said: "Light is light, regardless of who or what perceives it."

So now you don't think bees see yellow as purple? You agree that they see yellow as yellow? I'm not sure how they see purple - if you mean violet (around 400nm) then I guess they see it as violet. Alternatively if you mean a mix of red and blue light which would appear purple to us, they'd just see blue because they have no sensors to detect red light (most insects don't, except for butterflies apparently).

This suggests that "Light is light, regardless of who or what perceives it." isn't correct. Try it as "electromagnetic radiation is electromagnetic radiation regardless of who or what perceives it" - then see what your microwave oven does to a mug of water or a metal object.

and "Every colour you see is a wavelength reflected from the object." - unless the object emits it own light e.g. the sun, a lightbulb, a firefly's backside, etc. So if you shorten that to "Every colour you see is a wavelength" then I'd agree. And if that wavelength is around 580nm then the colour you see is yellow.

Bee-purple isn't my term, it was explained to me by an entomologist. To us it would appear yellow, but to a bee it would be an attractive colour (whereas yellow without the UV isn't if you're a bee apparently).

As for my local scientific dept. - do you mean a school or university dept? If the former then I'm submitting the idea to the person who gave it to me, if the latter I'm submitting it to myself. Both exercises seem pretty pointless.

Ok; I will put your minds at rest. I am mad. The so=called science I pursue on these forums is no more than a neural reaction beyween cels. You are right. The world is as you perceive it to be...

No, the world isn't as we perceive it to be - the demonstration with the red&green v orange LEDs demonstrated that. Different people (and organisms) perceive the world differently. And yet it's the same world that we're perceiving.