Why is the sun yellow?

[CalMeacham]

I’m really busy now, but I’ve looked through my books, and two sources tell me that about 20% of the direct sunlight gets scattered – which is much more than I thought. This scattered portion is more heavily scattered in the blue than in the red, the amount varying as 1/(wavelength)^4, the famous Rayleigh scattering law*.

Still, for practical purposes the light scattered by the particles in the sky is mostly single-scatter, and almost all the white light from the clouds you see is multiple-scattered sunlight. There will, indeed, be light from elsewhere scattering, but it’s going to be far lower intensity.

[Crafter_Man]

When children color an outdoor setting on paper, it is often done on white paper. A white crayon won’t show up on the paper. So they make the sun yellow.

[AWB]

So the Green Lantern Corps won’t move it?

[falcotron]

@AWB

So the Green Lantern Corps won’t move it?

How are they even going to find it, with all the times the Daleks and the Time Lords moved it?

[Mijin]

I saw a nice demo on a science programme where they shone white torchlight through a tank of cloudy liquid (I think it was water with a little milk mixed in).

The liquid would look bluish when the torch was held up close, and when looking at the torch directly through the tank, its light looked yellow.
Rayleigh scattering.

That said, I can’t seem to find a youTube clip of a (convincing) reproduction of this experiment. Perhaps it’s one of those misleading experiments, like weighing air using balloons on scales.

[CookingWithGas]

@Quercus

And perhaps also by contrast with artificial light, which tends to be more blueish than sunlight.

@dtilque

As for artificial light, the temperature of an incandescent light filament is somewhere around 3000 K (give or take a few hundred depending on wattage), so those are definitely yellower than the Sun. Fluorescent tubes are usually rated by their equivalent blackbody temp on the package. They range from about 3500 K to 5500 K or so.

Agree; conventional artificial light is yellow/orange. The difference between sunlight and tungsten-filament incandescent light is so much that conventional camera film for slides is (was?) available with a special corrected version for tungsten light if you shoot indoors. An alternative for print-film shooters is to use a blue filter, which compensates for the yellow/orange tint of the light bulbs. (Fluorescent light is a whole nother story.) The human eye, however, is not film, but rather a complex combination of sensors in the eyes plus what goes on in the brain. The brain tends to recalibrate light for conditions, sort of like auto-white-balance does in a digital camera. We still perceive a white piece of paper as white even if the ambient light is not white.

@Kinthalis

Ok, so does anyone actually have a regular old color photograph of the sun from space? Or are scientist always using xrays and radio waves to look at it?

For the reasons mentioned above, a photograph may not look the same as the perception of seeing the same scene in person. The film or digital sensor may not record true colors, or may record true colors without applying the corrections that the brain would. You can’t really prove much about how people see color by using photos as evidence.

[Spectralist]

I’m lost. How do these:

@Wikipedia

The Sun’s color is white, although from the surface of the Earth it may appear yellow because of atmospheric scattering of blue light.[12][13] Its stellar classification, based on spectral class, is G2V, and is informally designated a yellow star, because its visible radiation is most intense in the yellow-green portion of the spectrum.

Two sentences not contradict each other? If the sun gives more visible radiation in the yellow-green portion of the spectrum how can it be considered “White”?

[BrotherCadfael]

Just for the fun of it, I’m going to post this bit about Clifford Stoll’s thesis defense. He has confidently answered a variety of highly technical questions from his committee. The the last examiner speaks up:

@The Cookoo's Egg

“I’ve got just one question, Cliff,” he says, “Why is the sky blue?”

My mind is absolutely, profoundly blank. I have no idea. I look out the window at the sky with the primitive, uncomprehending wonder of a Neanderthal contemplating fire. I force myself to say something - anything. “Scattered light” I reply. “Uh, yeah, scattered sunlight.”

“Could you be more specific?”

Well words came from somewhere, out of some deep instinct of self-preservation. I babbled about the spectrum of sunlight, the upper atmosphere, and how light interacts with molecules of air.

“Could you be more specific?”

An our later, I’m sweating hard. His simple question - a five-year-old’s question - has drawn together oscillator theory, electricity and magnetism, thermodynamics, even quantum mechanics. Even in my miserable writhing, I admired the guy.

[CookingWithGas]

@Spectralist

I’m lost. How do these:

@Wikipedia

The Sun’s color is white, although from the surface of the Earth it may appear yellow because of atmospheric scattering of blue light.[12][13] Its stellar classification, based on spectral class, is G2V, and is informally designated a yellow star, because its visible radiation is most intense in the yellow-green portion of the spectrum.

Two sentences not contradict each other? If the sun gives more visible radiation in the yellow-green portion of the spectrum how can it be considered “White”?

Well, it is Wikipedia.

[UTejas]

@Mijin

I saw a nice demo on a science programme where they shone white torchlight through a tank of cloudy liquid (I think it was water with a little milk mixed in).

The liquid would look bluish when the torch was held up close, and when looking at the torch directly through the tank, its light looked yellow.
Rayleigh scattering.

That said, I can’t seem to find a youTube clip of a (convincing) reproduction of this experiment. Perhaps it’s one of those misleading experiments, like weighing air using balloons on scales.

What you describe sounds like an effect of The Tyndall Effect, which relates to colloids, such as water with some flour mixed in.

[Irishman]

falcotron said:

The sun’s surface is around 5700K. (Actually, 5 different search results gave 5 different values, from 5500 to 5820. 5700 is a nice round number close to the average of those 5.)

The reason you get different numbers at different sites is because each page is using an approximation. The Sun is not a perfect black body emitter. The values are chosen by trying to fit a perfect black body curve over the Sun’s spectrum and guess at the “best fit”. Everybody makes different variances in what is “best”, ergo you get different values.
ethansiegel said:

But there are two things further that go on here: one is that your eyes are better at seeing discerning mid-range and long wavelengths (greens, yellows, oranges and reds) than they are at short wavelengths (blues and violets). Want proof? Look at a rainbow; you’re way better at seeing the different shades of red than you are at, say, finding “indigo.”

Bad example. I don’t disagree that eyes and brains have preferential response, but the rainbow example is way off. You’re thinking of ROYGBIV: Red, Orange, Yellow, Green, Blue, Indigo, Violet. Except that Indigo isn’t really a significant color the same way that the other colors are. The problem is that Isaac Newton was a bit of a mystic. He did not want 6 major colors for the spectrum, because of numerology - 6 is the devil’s number. So to make 7 colors, he decided to include a shade between blue and violet. But if you look at primary and secondary colors, you get Red, Yellow, Blue; Orange, Green, Violet. There’s no room for Indigo. It’s a shade of blue, like Midnight or Sky. You say it’s hard to pick out Indigo from the rainbow. Well yes, but it’s also hard to pick out Aqua. Aqua is the mix of primary Blue and secondary Green (making it tertiary?). Indigo is mixing primary Blue with secondary Violet.

I’m not suggesting that violet light is made up of red light and blue light, I’m referring to color theory to show how there are different levels for describing colors. Indigo is not on the same level as Blue or Violet. That is why it is a lot less distinct.

CalMeacham said:

As I stated there, I strongly suspect the reason that we perceive the sun as yellow is that, as the sun descends and atmospheric scattering starts to shift its color towards the red you see when the sun is at the horizon, the sun begins to color yellow and stays that way for most of its descent.

Enilno said:

Part if it may be that most of the times when we can look at the Sun without it being too painful is during dusk and dawn, when the light has to pass through more of the atmosphere. So our mental imageof the sun is more red/yellow than it actually is.

Agreed.

ethansiegel, thanks for the link on blackbody.

[Malacandra]

@Irishman

Bad example. I don’t disagree that eyes and brains have preferential response, but the rainbow example is way off. You’re thinking of ROYGBIV: Red, Orange, Yellow, Green, Blue, Indigo, Violet. Except that Indigo isn’t really a significant color the same way that the other colors are. The problem is that Isaac Newton was a bit of a mystic. He did not want 6 major colors for the spectrum, because of numerology - 6 is the devil’s number. So to make 7 colors, he decided to include a shade between blue and violet. But if you look at primary and secondary colors, you get Red, Yellow, Blue; Orange, Green, Violet.

Yellow isn’t a primary colour of light, nor green a secondary. (You’re right about Newton’s thing about indigo, though.) There’s a reason why the colour controls for your computer are RGB, and printers use CMYK. That said, pure spectral yellow light isn’t a mixture: Shine a sodium light through a prism and you’ll see two bright yellow bands, nary a red or green to be seen. It’s just that your eye can only see it as red and green, and post-processes this to mean “yellow”; that’s why a colour screen works at all. Point a video camera at a sodium light, feed the signal to a computer or TV *, and split the spectrum: a pure yellow source still turns red and green by the time video processing has got at it.

• We omit here recent developments that use a RGBY screen for more vivid yellows.

[falcotron]

@Irishman

The reason you get different numbers at different sites is because each page is using an approximation. The Sun is not a perfect black body emitter. The values are chosen by trying to fit a perfect black body curve over the Sun’s spectrum and guess at the “best fit”. Everybody makes different variances in what is “best”, ergo you get different values.

Thanks.

Doing a little more reading, I think I understand better now. My second question (why the peak is bluer than you’d expect from the surface temperature) was basically meaningless and doesn’t have an answer.

And it’s starting to look like for my first question (why the image of the sun is yellower than daylight), maybe we just don’t know the answer. At least it seems like nobody can agree on one.

[Francis_Vaughan]

Weighing late into the question. A slightly different take.

The most important thing is to realise that there is no such thing as colour as an intrinsic quality of sunlight. Colour is entirely something made up in our heads. Our eyes are not spectral analysers, and have a very poor, and peculiar way of providing a very very basic additional measure of spectral distribution. One that is dependant upon they way the eye detects light and how the detected values are processed. First and foremost, we don’t see in red blue and green light. Even that is post processed from what our eyes do see. Indeed, when you look at how our eyes work, it is somewhat astounding we see anything like as rich a palate as we do.

The usual way of describing a colour is in the CIE colour space, where you get a two valued tuple that defines a point in colour space. There is a defined formula for taking any spectra and determining the perceived colour we see from that spectra. (Essentially for each 5nm wide band there is a contribution towards how much each of the three colour bands detects, this is then folded into the final claulation of location in coordinate space.) There are an infinite number of spectra that yield the same perceived colour. This is important.

Pure white is located at (0.33 0.33) in the colour space. If you take the spectra of sunlight you get a slightly different value. Actually you get a range of values, depending upon conditions.

The CIE standard defines a range of sunlight conditions (illuminants), from high noonday sun to more common forms of sunlight. Each of these has a different spectra, for the reasons discussed earlier here. The current standard illuminant is D6500 - an equivalent 6500K black body modified to include all the effects discussed earlier, making it look like daylight.

Since we know from first principles the spectra a black body has, we can always find the location in colour space of a black body at any temperature. For a very limited set of colours is possible to reverse out the effective black body temperature that would create that colour. Thus we see light sources that are not black bodies being assigned colour temperatures. The technique does not attempt to fit a black body curve around the spectra, rather you compute the CIE value for whatever raggedy spectra you have, and then look for the near match black body. In particular, this is how florescent lamps get their (arguably bogus) colour temperature values. You can have florescent lights with three simple peaks in their spectra, bearing no relationship to a black body spectra at all, and yet they get given a colour temperature. When you use them in practice you soon realise how bogus the claim is.

[falcotron]

@Francis_Vaughan

The most important thing is to realise that there is no such thing as colour as an intrinsic quality of sunlight.

Someone else already brought this up (in less detail), but it probably isn’t really relevant.

You can rephrase the original question as “why is the CIE value for the image of the sun not the same as that for daylight?”, and that certainly helps to clarify the question, but it doesn’t necessarily get us any closer to the answer.

Obviously it’s possible there could be an argument that appeals to CIE color space. In fact, you could easily rule out one of my early suggestions (which has already been dismissed) in this way. I suggested that maybe the direct image is effectively the same as daylight but more so, which causes us to see it as yellow. That’s the same as saying D6500 plus more Y is yellow, which is clearly not true from a quick glance at the CIE-xyY graph, so it’s wrong.

But at the moment, there seem to be two main contentions, and I don’t think the color space matters for either.

Are they actually different colors at all? Some have suggested that daylight and the sun’s image are essentially the same optically, and it’s only later in our processing (contrast effects in the higher visual centers, selective memory because we generally only look at the sun near dusk and dawn, or concepts being affected by convention) that makes us to think of them as different colors. Remapping the color space won’t help answer this one.

If that’s not true, then there’s a different spectrum mix, and the direct image causes proportionately fewer S cones to fire than M and L, which causes the yellow-blue channel to fire more yellow, which causes us to say the sun is yellow. So the question here is, how do you explain the different spectrum mix? Is it scattering? If so, why don’t the clouds also look yellow? Putting things in CIE terms actually takes us farther from the answer here, not closer.

All that being said, it’s possible that we’re completely missing the key question, and whatever that key question is could best be phrased in CIE terms. But, if so, just raising that possibility still doesn’t tell us what we’re missing.

[Francis_Vaughan]

One simple answer as to why clouds don’t look yellow is that they are backlit. Thus they are always backlit by daylight not sunlight. For them the sky is always blue. They should not look yellow. You have to get very high, way above the altitude where clouds are usually found, before the sky ceases to be blue. Clouds backlit by other clouds are also backlit by this same colour.

The reason I bring up the CIE colour space is that it allows us to reason about perceived colour, not just spectra. So I think it takes us right up to the edge of the problem.

I think the answer to the question has actually been fully covered. The sky is blue. The additional light from the sky plus direct sunlight makes up daylight. The balance of this varies through the day and with other effects, but you always have the two components. Clouds are lit by the equivalent of pure D6500 light, since there is nothing between them and this effective source, so they look pure white. By simple subtraction the amount of additional blue present in daylight due to the sky is what isn’t present in direct sunlight. This gets you to a yellower colour. You could calculate the CIE location of sunlight this way. It isn’t quite the same as adding yellow, you need to subtract the entire spectrum of skylight.

[AHunter3]

@FluffyBob

When I look at the sun - something I really try to avoid, I have a lot of trouble telling what colour it is.

I went though a phase as a kid when I would stare directly at the sun long enough to resolve it head-on as a round disk. I didn’t continue to stare at it for 10 minutes or anything but I could do it and I laughed to myself at the silly people who said doing this would make you go blind.

Hmm, my eyes don’t work all that well now that I’m 51. Go figure.

Anyway, it looked really pale washed-out lemon-juice yellow to me.

[falcotron]

@Francis_Vaughan

One simple answer as to why clouds don’t look yellow is that they are backlit. Thus they are always backlit by daylight not sunlight… The sky is blue. The additional light from the sky plus direct sunlight makes up daylight…

Yes, this is basically the same thing I was saying, and others (including in an earlier thread). But CalMeacham (among others) has an argument against this (although he hasn’t dug up the numbers yet, because he’s busy). So, I don’t know that it’s fair to say that it’s already been fully covered.

@Francis_Vaughan

The reason I bring up the CIE colour space is that it allows us to reason about perceived colour, not just spectra. So I think it takes us right up to the edge of the problem… You could calculate the CIE location of sunlight this way. It isn’t quite the same as adding yellow, you need to subtract the entire spectrum of skylight.

OK, good point. It’s true that subtracting a blue wavelength (or adding a yellow one) spectrally isn’t necessarily the same as going less blue (or more yellow) in color. But I think in this case, we’re so far within the range where it is true that this isn’t relevant.

Sunlight = sun’s spectrum - blue scatter. Daylight = sunlight + blue scatter. (Maybe not all of it, but enough of it that it’s less yellow than sunlight.) Clouds are illuminated by daylight, not sunlight. None of the relevant colors are anywhere near the edges of color space. So the sun is yellower than both daylight and clouds.

However, if you want to know how much yellower the sun is than daylight, you have to go to CIE space. And this would actually be useful information for testing the whole thing. After all, if it turns out that it’s so little difference that you probably couldn’t perceive it–or much more difference than anyone actually perceives–then it’s wrong.

So, you’re right to bring it up.

Meanwhile, I don’t want to put words in his mouth, but I think what CalMeacham is (primarily) contesting is the idea that clouds are illuminated by daylight. And we’re no closer to an answer on that one.

[falcotron]

@AHunter3

@FluffyBob

When I look at the sun - something I really try to avoid, I have a lot of trouble telling what colour it is.

I went though a phase as a kid when I would stare directly at the sun long enough to resolve it head-on as a round disk. I didn’t continue to stare at it for 10 minutes or anything but I could do it and I laughed to myself at the silly people who said doing this would make you go blind.

Hmm, my eyes don’t work all that well now that I’m 51. Go figure.

Anyway, it looked really pale washed-out lemon-juice yellow to me.

Of course you were also pouring lemon juice in your eyes at the time to prove that wouldn’t make you go blind either, right?

Just so long as you never put salt in your eyes.

[Irishman]

[quote=“Irishman, post:51, topic:556976”]

falcotron said:
The reason you get different numbers at different sites is because each page is using an approximation. The Sun is not a perfect black body emitter. The values are chosen by trying to fit a perfect black body curve over the Sun’s spectrum and guess at the “best fit”. Everybody makes different variances in what is “best”, ergo you get different values.
ethansiegel said:
Bad example. I don’t disagree that eyes and brains have preferential response, but the rainbow example is way off. You’re thinking of ROYGBIV: Red, Orange, Yellow, Green, Blue, Indigo, Violet. Except that Indigo isn’t really a significant color the same way that the other colors are. The problem is that Isaac Newton was a bit of a mystic. He did not want 6 major colors for the spectrum, because of numerology - 6 is the devil’s number. So to make 7 colors, he decided to include a shade between blue and violet. But if you look at primary and secondary colors, you get Red, Yellow, Blue; Orange, Green, Violet. There’s no room for Indigo. It’s a shade of blue, like Midnight or Sky. You say it’s hard to pick out Indigo from the rainbow. Well yes, but it’s also hard to pick out Aqua. Aqua is the mix of primary Blue and secondary Green (making it tertiary?). Indigo is mixing primary Blue with secondary Violet.

I’m not suggesting that violet light is made up of red light and blue light, I’m referring to color theory to show how there are different levels for describing colors. Indigo is not on the same level as Blue or Violet. That is why it is a lot less distinct.

Malacandra said:

Yellow isn’t a primary colour of light, nor green a secondary…That said, pure spectral yellow light isn’t a mixture: Shine a sodium light through a prism and you’ll see two bright yellow bands, nary a red or green to be seen. It’s just that your eye can only see it as red and green, and post-processes this to mean “yellow”; that’s why a colour screen works at all.

Apparently you didn’t read my very next paragraph:

I’m not suggesting that violet light [or yellow light, for that matter] is made up of red light and blue light (or any other combination of other colors), I’m referring to color theory to show how there are different levels for describing colors. Indigo is not on the same level as Blue or Violet. That is why it is a lot less distinct.

We have a name for a broad category of color: Red; Orange; Yellow; Green; Blue; Purple (i.e. violet when talking about light, but it is the color “purple”).

Then we have narrower names for shades of color within those categories: mauve; burgundy; aqua; teal; emerald; canary; etc.

“Indigo” can best be described as a subset of Blue, not distinct from Blue.

Except for Newton, who wanted a seventh color. He could just as easily have picked yellow-green and called it “Splotz”. ROYSGBV