Why is there a lack of consistency in portrayals of the visible light spectrum?

[shiva999]

is that true that the colour of sky is blue or is it just the reflection of sea water

 

[vhbelvadi]

The sky is blue because of scattering of light.
Here's a webpage you can read more about this: http://www.sciencemadesimple.com/sky_blue.html

 

[jewbinson]

I've just read that article and it's very easy to understand.

I think the sea is blue because salt water is light blue and the size of the sea causes an accumulaton of light blue, which is dark blue. This is just a guess though.

The sea is often much darker than the sky so I don't think the sea is a reflection of the sky

 

[ZapperZ]

Salt water is light blue? Since when?

Try this at home!

Zz.

 

[jewbinson]

You can't see it in just a few cm^3 or m^3 of water. But perhaps in 10^6 or 10^8 m^3 of water you will begin to notice a pale blue. As I say, it is very, very pale.

Aha! Look what I found

http://en.wikipedia.org/wiki/Color_of_water

 

[jewbinson]

So not just salt water, pure water is blue

 

[ZapperZ]

You have to be careful and understand what you are reading. There's a difference between the color of water seen through a transmission of light versus the reflection of that light. The FACT that the ocean's "color" can CHANGE, depending the time of day, the color of the sky, etc.. etc. (try sitting by the ocean for a long time and under different circumstances) means that it has very little to do with the color of the water... unless you think the color of the water itself changes that often.

Zz.

 

[vhbelvadi]

Right, so water is blue. But that's still not why the sky is blue.
Also, we can't entirely disregard the fact that part of the reason why large masses of water such as seas look blue is because of the reflection of the sky's colour.

 

[jewbinson]

But the sky's blue colour tends to be lighter than the colour of the sea (apart from seas in exotic Islands which can be very light blue).

When the sky is cloudy, the sea still tends to be quite dark blue.

 

[vhbelvadi]

I said 'part' of the reason, not the main reason! :-)
The question here was why the sky is blue, so I think we've got that solved well.

 

[thebiggerbang]

To debunk the OP, you can just assume that the guy observing the sky is sitting in an African dessert. Miles away from water! But the sky is still blue there! It's all about scattering!

 

[jewbinson]

What does there being no sea have to do with the sky being blue? The converse clearly has some relevance, because the sea reflects light. But the sky does not. Unless you were thinking about the sea "omitting blue light and the sky trapping the blue light". But that's got quite a few flaws to it

 

[ZapperZ]

But the sky's blue colour tends to be lighter than the colour of the sea (apart from seas in exotic Islands which can be very light blue).

When the sky is cloudy, the sea still tends to be quite dark blue.

But that already debunks even YOUR suggestion. I mean, how come the color of "salt water" changes when the sky is cloudy? Yet, one can easily argue that a "cloudy" sky causes the reflection to darkens.

At some point, responses to posts on PF must be based on more than just guess work. Re-read the https://www.physicsforums.com/showthread.php?t=414380" that everyone has agreed to, especially our policy on speculative post. If not, this thread is in danger of being locked.

Zz.

 

[klimatos]

is that true that the colour of sky is blue or is it just the reflection of sea water

Obviously the color of the sky varies from day to night and also with changes in the weather.

That said, the blue color of clear daytime sky is a result of the shorter wavelengths of sunlight being scattered by the molecules of the air itself (Rayleigh scattering). The scattering takes place all over the sunlit hemisphere of the atmosphere; hence the blue light appears to come from all parts of the sky.

Air molecules are too small to reflect other light wavelengths, but larger atmospheric particulates can do this. This gives us the common reddish sunsets.

 

[Dr_Morbius]

Sunsets are red for the same reason that the sky is blue. When you look at the sun near the horizon the atmosphere preferentially scatters the blue light away from your eye more than the red light. That makes the sun look redder than usual. The affect is increased by the fact that when the sun is near the horizon you're looking through more atmosphere than when it's overhead.

 

[LostConjugate]

I can't take it any longer! Someone just tell jewbinson that the ocean is blue because the sky is blue.

 

[Drakkith]

I can't take it any longer! Someone just tell jewbinson that the ocean is blue because the sky is blue.

I don't think that's true though.

 

[LostConjugate]

I don't think that's true though.

Well yea at sunset the ocean is red.

 

[jewbinson]

I can't take it any longer! Someone just tell jewbinson that the ocean is blue because the sky is blue.

?

This simply isn't true. When the sky is cloudy, is the sea white?

At night, there is nothing to light up the ocean, and water does not emit light, so obviously you see black (apart from the reflection of the moon)

 

[LostConjugate]

?

This simply isn't true. When the sky is cloudy, is the sea white?

At night, there is nothing to light up the ocean, and water does not emit light, so obviously you see black (apart from the reflection of the moon)

The sea should be less blue if your part of the sky is covered in thick clouds, yes. Some blue still scatters from where the sky is blue perhaps.

 

[Drakkith]

Well yea at sunset the ocean is red.

If so that is only because there is no blue light to see anyways, so of course it wouldn't appear blue. You wouldn't call a white sheet of paper red simply because it is sunset and the only light hitting it is on the red end of the spectrum would you? The fact is that water IS blue. The color that it can APPEAR to be varies depending on the impurities in the water and of course the ambient light.

See here: http://en.wikipedia.org/wiki/Color_of_water

Edit: Let us not mix up the COLOR of an object with the wavelengths that it absorbs or passes. The color can vary with the ambient light but the latter cannot. Water will lightly absorb the Red end of the spectrum, giving it a Blue color.

 

[LostConjugate]

If so that is only because there is no blue light to see anyways, so of course it wouldn't appear blue. You wouldn't call a white sheet of paper red simply because it is sunset and the only light hitting it is on the red end of the spectrum would you? The fact is that water IS blue. The color that it can APPEAR to be varies depending on the impurities in the water and of course the ambient light.

See here: http://en.wikipedia.org/wiki/Color_of_water

Edit: Let us not mix up the COLOR of an object with the wavelengths that it absorbs or passes. The color can vary with the ambient light but the latter cannot.

Are you sure?

It says here:

The blue tint of water is an intrinsic property and is caused by selective absorption and scattering of white light.

Edit: Oh I guess that is the definition for the color of a molecule. Does that mean that the sky is blue as well then?

 

[Drakkith]

Are you sure?

It says here:



Edit: Oh I guess that is the definition for the color of a molecule. Does that mean that the sky is blue as well then?

In my opinion that question is a bit more complicated than one might think. The atmosphere passes the entire visible range of the spectrum. HOWEVER, it causes differing amounts of refraction along the visible spectrum. When I look through my telescope at a white star high in the sky, it appears white. However, when it is low on the horizon the star shifts around and wavers, and it changes colors between white and reddish and back and forth several times a second. This is because of the increased amount of atmosphere that is between the star and myself and because of the angle that the light is entering at.

I would say that the sky looks blue, but it only has a color if you consider a prism to have a color simply because you are looking into one of the refracted colors.

EDIT: Also, looking at the absorbtion spectrum of the atmosphere, it does absorb some light in all wavelengths. It looks like it absorbs a little more red light than other colors, so if it does have a color to it, I would say that it would be blue. However one would never be able to notice it unless you could look at a light source through many miles of air without it refracting. So we would never be able to tell with the naked eye. So, with regards to the NOTICEABLE reason why the sky looks blue it is because of scattering and refraction, not because the sky IS blue.

 

[LostConjugate]

In my opinion that question is a bit more complicated than one might think. The atmosphere passes the entire visible range of the spectrum. HOWEVER, it causes differing amounts of refraction along the visible spectrum. When I look through my telescope at a white star high in the sky, it appears white. However, when it is low on the horizon the star shifts around and wavers, and it changes colors between white and reddish and back and forth several times a second. This is because of the increased amount of atmosphere that is between the star and myself and because of the angle that the light is entering at.

I would say that the sky looks blue, but it only has a color if you consider a prism to have a color simply because you are looking into one of the refracted colors.

I guess it does come down to the difference between scattering and absorption / emission. The latter being the color of an object, the former being an illusion.

Smoke being carbon is black by absorption / emission and blue by scattering if white light is present.

 

[Drakkith]

Pretty much LC.

 

[klimatos]

Sunsets are red for the same reason that the sky is blue. When you look at the sun near the horizon the atmosphere preferentially scatters the blue light away from your eye more than the red light. That makes the sun look redder than usual. The affect is increased by the fact that when the sun is near the horizon you're looking through more atmosphere than when it's overhead.

I disagree. If it were simply a matter of the distance traveled, then red sunsets would not be more common after volcanic eruptions, forest fires, and other sources of aerosols; and less common in clean air. Moreover, reflection and scattering is just as likely to be toward the viewer as away. Finally, in the years I've spent at sea I have seen many sunsets when the sky is blue right down to the horizon.

Your interpretation seems to be shared by a number of online sites, but all of my meteorology texts attribute red sunsets to aerosols. That's what I used to teach, and I'll stand by it.

 

[Drakkith]

I disagree. If it were simply a matter of the distance traveled, then red sunsets would not be more common after volcanic eruptions, forest fires, and other sources of aerosols; and less common in clean air. Moreover, reflection and scattering is just as likely to be toward the viewer as away. Finally, in the years I've spent at sea I have seen many sunsets when the sky is blue right down to the horizon.

Your interpretation seems to be shared by a number of online sites, but all of my meteorology texts attribute red sunsets to aerosols. That's what I used to teach, and I'll stand by it.

A large part of the color of the sun and moon is due to the air quality. I'm sure it is possible to see blue sky right down onto the horizon, especially out on the sea where the air is most likely clearer than it is near the shore and on land. More atmosphere also means more impurities for the light to travel through and greater chance that the light is scattered. After an eruption or fire there is much more impurities in the air than normal, so much more blue light is absorbed than normal. Both refractive effects AND absorption through impurities changes the color you will see.

My post above explains the effect of refraction in the atmosphere. The changing properties of the air cause a constantly varying level of refraction, which is magnified by the angle and greater distance the light has to travel when something is low on the horizon.

 

[vhbelvadi]

To sum it up:
Water is by itself blue, but also partly as a result of reflection of the sky.
The sky is not blue, but looks so because of scattering. The sunset is also because of scattering but, given the distance and thickness of the atmosphere, only longer wavelengths reach the observer, making it appear to be of a colour more towards the red-end of the spectrum. As for aerosols, they are also responsible for an increased shift of red colouration creating an actual 'red' sky rather than a yellow or orange one as would have been in the absence of aerosols.

(I found more on aerosols, in this regard, here: http://www.webexhibits.org/causesofcolor/14B.html )

 

[Drakkith]

I cannot agree with any meaningful portion of the color of water being because of blue scattered light from the sky. The amount of light NOT scattered that does hit the water vastly outdoes the small amount of scattered blue light that does.

 

[vhbelvadi]

I cannot agree with any meaningful portion of the color of water being because of blue scattered light from the sky. The amount of light NOT scattered that does hit the water vastly outdoes the small amount of scattered blue light that does.

Agreed.
But the light that is not scattered wouldn't affect the sea's colour in the way you state simply because it is concentrated along the line from the observer to the sun (i.e. where the sun rays were most intense, because, unlike the blue-end of the spectrum, they can't spread out.) Therefore, the colour striking a larger area of the sea is still the scattered blue, making reflection partly the reason for the sea's blue colour.

 

[Drakkith]

Every point in the water is in direct line of sight with the sun. A MUCH greater proportion of light comes directly from the sun that it does from the scattered light. And if you think about it, if the light coming down gets scattered slightly, then the added scattered light plus the non scattered light should equal out anyways.

 

[vhbelvadi]

And if you think about it, if the light coming down gets scattered slightly, then the added scattered light plus the non scattered light should equal out anyways.

So reflection is a plausible explanation.

 

[jewbinson]

But when the sky is cloudy (on a bright day, i.e. NOT when there are thick, dark clouds), the sea is still blue.

I would say reflection is a partial explanation.

The water in swimming pools I think are pale blue even when the floor of the swimming pool is not blue (most swimming pools do have blue floors from memory)

 

[vhbelvadi]

I would say reflection is a partial explanation.

That's all we've been saying all along!
Reflection is only a partial explanation, but it's an explanation nonetheless; so it can't be entirely disregarded.

 

[jewbinson]

Yeah I think if the sky were brown or bright green or yellow or pink the sea would be a slightly different colour.

 

[vhbelvadi]

 

[jewbinson]

cool persuasive argument bro

 

[Claude Bile]

The term color needs defining here.

How do we define the color of an object?
By the transmission spectrum?
By the reflection spectrum?
By the spectrum we detect when we point a detector up at the sky?

The answer will be different depending on how you define color.

Claude.

 

[jewbinson]

Good point. I guess what is meant is "why, when we look up at the sky, do we see that it is blue?". The sky doesn't really have any "colour"

 

[vhbelvadi]

As always apparent colour!

 

[256bits]

Here is a picture of the Earth from space - Apollo 17

http://www.skyimagelab.com/ap17fulearim.html

The ocean looks blue.

 

[Drakkith]

We can go around in circles for days. Water WILL absorb small amounts of red light from a white light source that passes through it, causing it to look blue. It will also reflect light of all color if it hits it at the right angles. Trying to explain the color of water as being partially due to scattered blue light from the atmosphere is like trying to explain the color of the grass and everything else outside the same way.

I think this effect has been explained pretty well, so unless someone has something new to ask or add then I think this will be my last post here.

 

[cmb]

I may be presenting an opening gambit here that will get you thinking I'm not quite right, but I think I can evidence the following sufficiently to at least make a case;

The Sun is green [evidence: its peak wavelength is the same as the colour of green plants]
The ocean is green, it is full of the same chlorophyl pigment-bearing micro-organisms found in all other plants, and that match the peak emission of the Sun. [Take a photo of the sea with a ploarising filter set to vertical, so all the horizontal reflections are taken out of it.]
The sky is white, just the same colour as the clouds. The blue is an optical illusion - the blue receptors in the eye respond to an excess of UV stimulation making you think you're seeing blue. [evidence/thought experiment: take a white sheet of paper out in a rowing boat all painted sky blue. Dress head to toe in sky-blue clothing. What colour do you think the paper now looks? If the sky really was blue, why does white paper look white when shaded from the sun but otherwise under a fully blue sky? Answer - because the paper does not reflect the UV element of the spectrum, thereby avoiding the false stimulation of the eye making it think it is seeing blue.]

[Caveat - this is a straw-man proposition for you to cut down. I am not suggesting there isn't an element of blue in either the sea or the sky, because there is Rayleigh scattering going on, no doubt. But I do not think I have yet seen evidence to suggest it is the major part of us seeing blue.]

You may also ask - "hey, if the sky isn't actually blue, then why does my digital camera take the sky as 'blue'. Is my camera experiencing an optical illusion, too!?". Answer, yes. That's why there is a white balance setting - so that your camera can be made to take a picture like you expect it to look.

 

[Drakkith]

The Sun is green [evidence: its peak wavelength is the same as the colour of green plants]

The sun is white. The intensity of the output in the visual spectrum doesn't shift enough between colors to claim that the Sun is predominately green.

The ocean is green, it is full of the same chlorophyl pigment-bearing micro-organisms found in all other plants, and that match the peak emission of the Sun. [Take a photo of the sea with a ploarising filter set to vertical, so all the horizontal reflections are taken out of it.]

While I will agree that this has an effect on the color of parts of the ocean, as a whole the ocean is not that green.

The sky is white, just the same colour as the clouds. The blue is an optical illusion - the blue receptors in the eye respond to an excess of UV stimulation making you think you're seeing blue. [evidence/thought experiment: take a white sheet of paper out in a rowing boat all painted sky blue. Dress head to toe in sky-blue clothing. What colour do you think the paper now looks? If the sky really was blue, why does white paper look white when shaded from the sun but otherwise under a fully blue sky? Answer - because the paper does not reflect the UV element of the spectrum, thereby avoiding the false stimulation of the eye making it think it is seeing blue.]

I don't believe this is correct. First, IF the sky was perfectly transparent and didn't scatter light it would be black and you would be able to see the stars. Since it is not perfectly transparent it scatters light. The majority of the scattered light falls in the blue range of the spectrum, hence making it appear blue.

Second, in most people the eye blocks out UV light from reaching the retina. On top of that I believe that UV light that does strike the retina stimulates all types of color receptors equally, making UV light look white not blue.

Third, the amount of light scattered in the atmosphere is much less than the amount not scattered. Also, as I said above, if part of the blue light is being scattered out of the sunlight, then the non scattered light being reflected from the paper adds with the scattered blue light that also reflects off the paper and probably equals back out to look almost perfectly white again.

[Caveat - this is a straw-man proposition for you to cut down. I am not suggesting there isn't an element of blue in either the sea or the sky, because there is Rayleigh scattering going on, no doubt. But I do not think I have yet seen evidence to suggest it is the major part of us seeing blue.]

This effect is explained pretty well in the following link. I have never seen evidence to the contrary so I don't have any reason to not believe it.
See here: http://en.wikipedia.org/wiki/Diffuse_sky_radiation

 

[cmb]

This effect is explained pretty well in the following link. I have never seen evidence to the contrary so I don't have any reason to not believe it.
See here: http://en.wikipedia.org/wiki/Diffuse_sky_radiation

I'll overlay a colour spectrum over the 'spectrum of blue sky' given in the page you've just linked to.

More blue than green? I can't say I am entirely convinced?

For sure there is blue light scattering. I don't dispute it. That's why the Sun looks yellow - because the blue light is scattered out of the 'white light' (which peaks around green). I'm just interested to try to figure out how much of the blue we see is 'real' and how much is down to our perception of 'blue'. I'm certain it is not all from scattering, but 'how much of the perceived blueness' is scattering, I do not know.

The eye cannot see green very well. It generally sees 'red' & 'blue' ('L' and 'S' cone cells) and 'bright/dark' (rod cells). The 'M cells' that select for green don't seem to do a very good job and sensitivity is skewed towards responding similarly to the 'L' cells. The eye generally perceives deeper green (~520nm, like the Sun's peak colour) by seeing 'not red + not blue + bright' and the eye concludes that to be green. We can't see the 'Solar Green' colour directly, hence incandescent objects go from red to white to blue as they get hotter and we never perceive a green incandescence, but it seems absurd to say that there is never a 'green' incandescence. It is simply a continuum of spectrum, merely one we cannot perceive with our limits of colour vision. Red and blue are far enough from each other that we find them easy enough to distinguish. But green is a bit too close to either for our relatively poor colour acuity to discriminate.

You say you've never seen evidence otherwise. I'll put forward the paper experiment again - if you stand in the shade of a blue building and stand on a blue carpet, then if you look at a piece of white paper then surely it should look blue? Why does a piece of white paper not look blue under a blue sky, if the colours coming down from the sky are truly 'blue'?

 

[Drakkith]

Per the info when I clicked on that picture from the article:

Spectrum of blue sky somewhat near the horizon pointing east at around 3 or 4 pm on a clear day. Spectrum was taken using an Ocean Optics HR2000 spectrometer [1] with a high-OH solarization-resistant fiber optic light guide. this spectrum is NOT BY ANY MEANS IDEAL and was taken from inside a laboratory through probably 4 panes of window glass, thus completely attenuating virtually all radiation below 400nm also the end of the fiber optic was not coupled to any collimating optics thus there may be some slight skewing of the spectrum due to diffuse reflections off surrounding buildings and trees etc. Because the response of the CCD detector in the spectrometer is not linear the spectrum in the infrared region is also less than what is actually present in sunlight; the blackbody spectrum of sunlight continues much further into the infrared than is shown here. This spectrum is not calibrated for intensity.

Also, to my knowledge, the best response for the eye in sunlight is the Green/Yellow area, not its worst. (EDIT: I see what you mean about the response of the green cone cell being very close to red, but that is not part of the issue here.) Also, the effect of temperature on light emitted is very different than what you are describing. As the temperature increases, more light at a lower wavelength is emitted. For the visual spectrum an object continues to emit lots of red and orange light even as the temp increases and it emits shorter wavelengths. So we would never see green by itself because it is right in the middle and we see white instead. When it gets hot enough that it becomes Blue, the object is emitting much more light in the blue and UV range and the lower red/green portion has fallen off.

You say you've never seen evidence otherwise. I'll put forward the paper experiment again - if you stand in the shade of a blue building and stand on a blue carpet, then if you look at a piece of white paper then surely it should look blue? Why does a piece of white paper not look blue under a blue sky, if the colours coming down from the sky are truly 'blue'?

Are you sure it doesn't look blue? Your example has so many variables that I can't possibly say anything for sure on it. Ambient light isn't just dependent on scattered atmospheric light for one thing.

Edit: I will agree that the response of the cone cells in your eye definitely determine how we differentiate color. But in the end the effect is the same. We "see" blue because that end of the spectrum is scattered more than the reddish end. In regards to the topic, I think we should avoid the issue of how color vision works and stick to wavelengths of light and the effects of the atmosphere on them.

 

[cmb]

We "see" blue because that end of the spectrum is scattered more than the reddish end. In regards to the topic, I think we should avoid the issue of how color vision works and stick to wavelengths of light and the effects of the atmosphere on them.

If you want to address a question related to 'colour' then you have to address how the eye sees it. 'Colour' is purely, and completely a human perception.

To be objective about it, you'd have to say something like;

"The light from the sky is similar to a black-body radiator with a flattened emissions' peak centred around 500-520nm, which is a wavelength perceived by humans as 'blue-green'." I don't see how you can be more objective than that.

The atmosphere does tend to filter (and scatter) the blue content of the solar radiation, as given in the figure below (sorry, I do not recall the reference I got this from. I added the colour spectrum for reference).

You can see there is a stack of near-UV and UV that is filtered and/or selectively scattered, but overall we still appear to end up with more green wavelengths at sea level than any other 'colour'.

Also, to my knowledge, the best response for the eye in sunlight is the Green/Yellow area, not its worst.

I surely didn't say it was the worst sensitivity. It is the worst region for discriminating wavelengths.

There is a very simple reason why the rods (that cannot perceive colour)* are most sensitive to ~500nm wavelength. It's because that is the wavelength most prevalent on the surface of the earth! In dark conditions, this is still true (the prevailing light, if any, is still the Sun's) so the eye has evolved to pick up every available photon, and it is obviously going to evolve tending towards a peak of sensitivity at the wavelength of the commonest photons.

*(we see only black and white in the dark because only the rods are sensitive enough to work there, which is why our brain isn't wired to use the rods to discriminate colour. So around the wavelength of the rods greatest sensitivity we discriminate colour badly. My interpretation of all of this is that the rods contribute significantly to our perception of green; if the rods detect 'bright' and the cones detect low 'red' and 'blue' we see that as green. But in white light there is also lots of red and blue, so the eye simply calls that 'white' as it can't resolve any differences for peaks around green.)

I realize it might be a 'shot-out-of-the-green' to realize 'blue sky' might be a perceptual illusion, but think it over and the idea might warm on you after a while!

(PS The sky looks blue to me, too!)

 

[Stcloud]

The sky is not blue, the grass is not green.. the colour is in us. Please see 'Physics Light'
and from there be directed to 'colour'.

 

[Drakkith]

The sky is not blue, the grass is not green.. the colour is in us. Please see 'Physics Light'
and from there be directed to 'colour'.

Yes and no. The sky scatters and absorbs certain wavelengths, and the way our eyes and brain process color results in us seeing it blue even though the wavelengths fall over a blue and greenish range. A green plant absorbs reddish and blueish light and so looks mostly green to us. This of course is assuming an average person with no injuries or abnormalities with their vision. A color blind person would not be able to discriminate colors as well, and the way things look would change. There is already a thread in process about color vision, so I REALLY don't want to get into the nitty gritty details in this one as well even though the two topics are related.

I realize it might be a 'shot-out-of-the-green' to realize 'blue sky' might be a perceptual illusion, but think it over and the idea might warm on you after a while!

Yes, most of the light that you see from a blue sky is in the blue-green area. It looks mostly blue because of the way our cone cells and brain work to give us color vision. HOWEVER blue light is scattered more than green light still, just as violet light is scattered more than blue. I have never claimed that more light reaches your eye in the blue range than the blue-green range.

You can see there is a stack of near-UV and UV that is filtered and/or selectively scattered, but overall we still appear to end up with more green wavelengths at sea level than any other 'colour'.

Yes, that is because more of the blue and violet light is scattered and absorbed than green and below. Note that the graph is talking about light from the sun hitting the Earth without being scattered or absorbed. Since more blue and violet light is missing, the sun appears yellowish when you look at it instead of white.

My interpretation of all of this is that the rods contribute significantly to our perception of green; if the rods detect 'bright' and the cones detect low 'red' and 'blue' we see that as green. But in white light there is also lots of red and blue, so the eye simply calls that 'white' as it can't resolve any differences for peaks around green.

This is incorrect. A rod cell is desensitized after it has been exposed to a large number of photons. It does not contribute to color vision at all. Rod cells aren't even wired the same way that cones are. They are grouped up with many rod cells converging onto a single nerve so that when multiple cells activate the signal is added together to give a greater response at the cost of resolution. This results in rod cells enabling to see in very low levels of light that we would otherwise be blind in.

White light looks white because it is stimulating all our cone cells at the same time. The combined input is filtered and interpreted by the brain and is perceived as white.

 

[cmb]

The sky is not blue, the grass is not green.. the colour is in us. Please see 'Physics Light'
and from there be directed to 'colour'.

Indeed. Colour is a human 'perception'.

But we can be a little more sophisticated than simply throwing our hands up and not looking further at particular scenarios.

I believe the Horizon programme (in the thread you reference) reports recent work on perception of, particlarly, 'grey' shades. Clearly, as I describe above, if the eye is making guesses about near-green colours because there is a hole in its perceptual ability there, then it may interpret a grey to be anywhere around green (or immediate neighbouring spectral colours), according to context, because that is how the eye works. Can't see green? Then maybe greens look white/grey, and white/greys can look green.

Another effect is where the eye sees a shadow under illumination of a solid colour. It's not evolved to see a full spectrum of such shadows, so again can get confused because it is expecting a blue-green dominated light spectrum, and sees shadow presuming the context is blue-green illumination.

The issue of the blue sky is different to these two. It appears to be over stimulation of the S cells by UV, because they have sensitivity right through to 400nm*, but is related to the first in that if it is 'only green' then the eye 'allows itself' to be easily fooled because it just doesn't 'do green' very well. It'll take other cues preferentially, if there are any, and allow them to over-ride a perception of green.

*(viz. if the eye is still 50% sensitive to 420nm than at 450nm, so if there is a stack of 400-420nm energy bundled into light that is strong in all wavelengths, then it'll tend to perceive that additional 'violet' as shading the white light towards strong blue. Caveat; again, I will repeat that this is a contribution to the effect of 'blue sky' but there is, agreed, a general shift towards blue-green due to scattering. It is the ratio of those two effects on the final perception of blue I do not believe is known or well understood.)