1145: "Sky Color"

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StClair
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Re: 1145: "Sky Color"

Postby StClair » Tue Dec 11, 2012 5:55 am UTC

Alltat wrote:It's not really much of a chicken-or-egg situation:

That is, of course, correct and I chose my words/simile poorly. Apologies.
Rather, I simply meant to point out why the visible spectrum is the visible spectrum, for eyes evolved to see by what a G2 star emits and a nitro-oxy atmosphere is transparent to.

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Bernkastel
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Re: 1145: "Sky Color"

Postby Bernkastel » Tue Dec 11, 2012 7:53 am UTC

Another comic where the alt-text turns out to be way more interesting than the rest of the comic...

Even knowing that mirrors flip front and back, it still took me a lot of thinking, and me reading through most of the page 1 posts to understand it.

The text in a book appears mirrored because you mirrored the text yourself when flipping the book. The mirror just makes it easier to see the flipped text. As someone pointed out, if you place one of the pages in front of a powerful light so you can see through the page, the text will still be flipped, even though you're not using a mirror to read it.

This also explains why the text doesn't get mirrored upside-down. It's because when you turn the book to face the mirror, you flip it horizontally, not vertically.

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Re: 1145: "Sky Color"

Postby marsgreekgod » Tue Dec 11, 2012 8:30 am UTC

So my question is, if we got a very strong purple light in space, could we turn the sky purple?

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Re: 1145: "Sky Color"

Postby honnza » Tue Dec 11, 2012 8:46 am UTC

marsgreekgod wrote:So my question is, if we got a very strong purple light in space, could we turn the sky purple?


Sure. This would also cause global warming, so beware.

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Re: 1145: "Sky Color"

Postby stianhat » Tue Dec 11, 2012 9:00 am UTC

The sky is blue because of the colors it *lacks* not the colors it has - we percieve chlorophyll filled leaves as green because they lack the red light and correspondingly, as someone was close to earlier, we see the sky as blue because it lacks red light, but also has violet light ( violet = not yellow ) (not red = green). the resultant mix of (not yellow) + green = blue. The blue light that is already there just adds to this.

And the mirror thing; it is a mirror *plane* located outside the image it is mirroring. It will switch the left and right directions and the forward / backwards one but leave up / down. Put the mirror on the floor and look at it, up and down now switched, left and right is switched but forwards / backwords is now persistent between the real image and mirror image.

Mirror planes are always make the mirror image and entity symmetrical along the mirror plane but switches direction on the other planes.

BTW dont use yourself as the mirror object, it will only confuse you as you seldom percieve your true image (just the mirror one) =P

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Re: 1145: "Sky Color"

Postby Solarn » Tue Dec 11, 2012 11:32 am UTC

Lawton wrote:I almost forgot, left and right are special, as there is no possible way of defining them without left and right already being established. (just imagine trying to define "left" to someone over radio)

"Okay, stand up straight and be very still. Try to concentrate on your heartbeat. Can you feel it? The side you feel it on is the left side." It's not perfect (apparently some people have their hearts reversed so it pumps more strongly on the right side), but it's good enough.

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Re: 1145: "Sky Color"

Postby Red Hal » Tue Dec 11, 2012 11:55 am UTC

Roses are Red
Violets are too,
If they move quick enough
away from you.

Roses are red
Violets are violet
please don't poke that
snake you'll just rile it.

Roses are black
Violets are white
Under appropriately
Chosen light.
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Re: 1145: "Sky Color"

Postby domenox » Tue Dec 11, 2012 12:52 pm UTC

The sky color question perplexed me and I've found myself looking for various answers all throughout the net and have come across a variety of answers from people, most of them saying it's due to a combination of factors with the final factor being the way our eyes work or how the eyes have evolved.

So I got to wondering why even a camera would be able to capture a blue sky as a blue sky. Well, some other people have asked that as well and the answer proposed by others was due to the camera capturing and merely relaying the same image that our eyes see, therefore it would still look to our eyes as blue but isn't necessarily, empirically blue.

But then, I got to wondering maybe a digital camera would be able to answer whether the sky is "actually" blue and rule out the hypothesis that our eyes are a big factor when it comes to seeing blue skies? Couldn't the data corresponding to pixels of a blue sky be compared to the data corresponding to a sample violet pixel and could this then answer whether a sky is "really" blue or we're just perceiving it wrongly as blue due to our eyes' limitations? (I'm not knowledgeable on cameras or optics or anything so just tell me if I'm missing something obvious here like for instance maybe digital cameras are created to match our eyes' color judgment abilities or whatnot.)

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Re: 1145: "Sky Color"

Postby Harry Voyager » Tue Dec 11, 2012 2:18 pm UTC

Jorpho wrote:
mrb4 wrote:Yet another unnecessarily complex and long answer to "why is the sky blue", *sigh*... The answer is simply:

Blue is the color of oxygen. (Yes, gaseous oxygen is very, very slightly blue, to a point it is sometimes incorrectly described as colorless.)
The next obvious question then is, how can the sky be blue if the atmosphere is only 21% oxygen?

xtifr wrote:Technically, all that stuff about scattering is true, but it completely misses the point, and ignores the fact that a nitrogen atmosphere would not be blue.
How so? Rayleigh scattering just depends on the presence of particulate matter, does it not? Even http://amasci.com/miscon/miscon4.html#blue says as much and places no particular emphasis on the presence of oxygen. As I said earlier, skim milk will appear blue under the right conditions, and veins also appear blue due to scattering through the skin.


Nitrogen is colourless, so the oxygen is a diluted dye. The atmosphere is also 11km deep, and 5*10^18 kg, so if I've done the numbers right, if you are standing on the ground, looking straight up at space, you are looking through something like 40 gigatonns of atmosphere. Even a very dilute dye can acquire an intense colour when you look through enough of it.

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Re: 1145: "Sky Color"

Postby aLuZiNK » Tue Dec 11, 2012 2:19 pm UTC

The second question is a trick question. Our two eyes are positioned horizontally. If you turn your head in 90 degree the image will appear vertically.

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Re: 1145: "Sky Color"

Postby Jorpho » Tue Dec 11, 2012 2:24 pm UTC

domenox wrote:But then, I got to wondering maybe a digital camera would be able to answer whether the sky is "actually" blue and rule out the hypothesis that our eyes are a big factor when it comes to seeing blue skies? Couldn't the data corresponding to pixels of a blue sky be compared to the data corresponding to a sample violet pixel and could this then answer whether a sky is "really" blue or we're just perceiving it wrongly as blue due to our eyes' limitations? (I'm not knowledgeable on cameras or optics or anything so just tell me if I'm missing something obvious here like for instance maybe digital cameras are created to match our eyes' color judgment abilities or whatnot.)
That would amount to the spectrograph seen here, if I understand you correctly:
Barstro wrote:Image


stianhat wrote:The sky is blue because of the colors it *lacks* not the colors it has - we percieve chlorophyll filled leaves as green because they lack the red light and correspondingly, as someone was close to earlier, we see the sky as blue because it lacks red light, but also has violet light ( violet = not yellow ) (not red = green). the resultant mix of (not yellow) + green = blue. The blue light that is already there just adds to this.
Chlorophyll absorbs red and violet light such that the only light it reflects is green, which we perceive. But the sky is blue because the blue end of the spectrum is more widely scattered than the other wavelengths. The color wheel doesn't really get into it, I think.

Harry Voyager wrote:Nitrogen is colourless, so the oxygen is a diluted dye. The atmosphere is also 11km deep, and 5*10^18 kg, so if I've done the numbers right, if you are standing on the ground, looking straight up at space, you are looking through something like 40 gigatonns of atmosphere. Even a very dilute dye can acquire an intense colour when you look through enough of it.
Let's put it this way, then: why is oxygen blue?

It is hinted in http://www.youtube.com/watch?v=WuG5WTId-IY , posted earlier, that the color of liquid oxygen results from the solvated, unpaired electron. A better example would be the Birch reduction (video), in which sodium (and its easily-removed outer electron) is dissolved in liquid ammonia. Basically, the molecules align in such a way as to confine the movement of the electron, and the confined electron can absorb the relevant wavelengths of visible light to reach an excited state.

But in its gaseous state, the oxygen molecules would be too far apart to confine an electron in a similar way. So why would oxygen be blue? If I am not mistaken its absorption spectrum lies entirely beyond the visible range.

EDIT: Okay, that sounded plausible, but a little more Googling suggests that isn't right either. http://chemed.chem.purdue.edu/demos/mai ... /14.2.html puts it quite plainly:
A single photon carries enough energy to excite two O2 molecules simultaneously. This transition is not observed in small amounts of oxygen gas at low pressures due to the very low probability of this three-body process. In the liquid, however, this transition is rather common because, as a general rule, the volume of a gas decreases by a factor of about 800 when it forms a liquid.


More to the point, even the Wiki on oxygen says it is a colorless gas (as opposed to ozone, which is plainly described as pale blue).
Last edited by Jorpho on Tue Dec 11, 2012 2:33 pm UTC, edited 1 time in total.

Geekoid
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Re: 1145: "Sky Color"

Postby Geekoid » Tue Dec 11, 2012 5:19 pm UTC

Another case of a physicists thinking something is trick becasue it's outside their experience. Why are physicists the worst at that?
It's simply the way the eyes perceive that mix of those frequency.

To be clear, everyone does it, but physicists are so arrogant about it.

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Re: 1145: "Sky Color"

Postby Jorpho » Tue Dec 11, 2012 5:29 pm UTC

Why stop there when you can simply say "Because God made it that way" ? :P

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Re: 1145: "Sky Color"

Postby domenox » Tue Dec 11, 2012 5:46 pm UTC

Jorpho wrote:That would amount to the spectrograph seen here, if I understand you correctly:


Yes, so if in case a spectograph registers the sky as "actually" blue, then we could perhaps rule out our perceiving the sky as blue as due to a quirk/fault in how humans' eyes/sight work and look instead to other possible reasons?

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Re: 1145: "Sky Color"

Postby orthogon » Tue Dec 11, 2012 5:49 pm UTC

benefluence wrote:The way I've always though about the mirror one is that what the mirror actually does (a reflection across the plane of the mirror) is equivalent to a 180 degree rotation about the vertical axis of the mirror followed by a left-right reflection. We perform the rotation subconsciously because we see the reflection as a person standing in front of us, and that's how we would get behind the mirror if such a thing were possible. Up and down ARE flipped, if you think about 'getting behind the mirror' as rotating around the horizontal axis parallel to the mirror, as such a rotation followed by an up down reflection is also equivalent to the mirror's reflection. we just don't think of it that way.


I would like to upvote this as my favourite explanation, and also because it invites the delightful mental picture of the other way of getting behind the mirror, which would be a sort of reverse-Fosbury-flop-into-headstand which I would like to see somebody perform. Internet points to anybody who can furnish me with a Youtube link.

[Edit] ... but please don't actually try it at home, kids. I wouldn't want that on my conscience.
xtifr wrote:... and orthogon merely sounds undecided.

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Re: 1145: "Sky Color"

Postby cream wobbly » Tue Dec 11, 2012 7:03 pm UTC

mikrit wrote:
rhomboidal wrote:If the sky was violet, the poem would go "roses are red, violets are violet," which would be redundant instead of romantic, and eventually the entire species would go extinct because of its crappy love poetry.

Tell me why the stars do shine,
Tell me why the ivy twines,
Tell me why the skies are blue,
And I will tell you why I love you.

Nuclear fusion makes stars to shine,
Tropisms make the ivy twine,
Rayleigh scattering make skies so blue,
Testicular hormones is why I love you.

Second verse published but not written by Isaac Asimov.

(It's like this: Asimov cited it in his Treasury of Humor when he didn't know the author. In a later book, though, he had learned the author's name and gave him credit. But I cannot remember the name and I have returned the book to the library.)

It says in X Stands for Unknown that the last line was Glandular hormones. But anyway, I can't push the Google Books search any further down the page, drat it.

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Re: 1145: "Sky Color"

Postby Pfhorrest » Tue Dec 11, 2012 7:25 pm UTC

orthogon wrote:I would like to upvote this as my favourite explanation, and also because it invites the delightful mental picture of the other way of getting behind the mirror, which would be a sort of reverse-Fosbury-flop-into-headstand which I would like to see somebody perform.

I don't see why anything kind of convoluted gymnastics would be necessary. I think of it like this:

If you could step through the mirror and turn around, normally you would do so by putting one foot forward through the mirrorplane, then pivoting along the axis of your spine as you bring the other foot through. You wouldn't end up like the mirror-you you normally see though: you would have to be flipped along the axis of your hips in order for that to happen.

Alternately, you could step through the mirror by reaching through the mirrorplane with your hands, then bending forward around the axis of your hips into a handstand to bring your feet through. You wouldn't end up like the mirror-you you normally see though, only in this case that's even more obvious: you're upside-down, and you would have to be flipped along the axis of your spine in order to end up like mirror-you.

The book example works the same. Hold a book up to a mirror as though you were letting mirror-you read it. If you could push the book through the mirrorplane into the same place as the mirror-book, you would ordinarily push it through the plane, and then rotate it along its spine. But that wouldn't end up looking like the mirror-book; you'd have to flip it horizontally to get that. So we think the mirror-book is "flipped horizontally".

But you could also get the book into the mirror by pushing it through, then rotating it head over heels until it faces you. Of course then it would be upside-down, and so would have to be flipped vertically to look like the mirror-book usually does. So you could just as well think of the mirror book as "flipped vertically" if you liked.

It's only because we intuitively don't let things get upside-down that we imagine that the things we see in the mirror "got there" by turning around a vertical axis, and then got flipped on a horizontal axis to end up as we see them. They very well could end up in the same state by being rolled head over heels, and then flipped vertically; the end result would be the same.
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Re: 1145: "Sky Color"

Postby orthogon » Tue Dec 11, 2012 7:59 pm UTC

Agreed. I was assuming you couldn't step through the mirror, what with it being made of glass, so you had to go around it or over the top. Hence the acrobatics.
xtifr wrote:... and orthogon merely sounds undecided.

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Re: 1145: "Sky Color"

Postby domenox » Tue Dec 11, 2012 8:36 pm UTC

Regarding the mirror thing, maybe it's not even necessary to think of "mirrors" to get the left-right reversal effect. Think of 2 identical bald twins facing each other. Or, say, when your palms face each other, or even when you fold paper with a symmetrical design on it. Maybe it's all just a similar effect to the folding of a 2 dimensional plane (like paper).

But, I think when you're looking at a mirror, up is still up. And down is still down. But if we rename "left" and simply call it "there" and we start calling "right" as "the opposite of "there"" then we will notice that when we look at the mirror, "there" is still "there" in the same way that up is always still up. Conversely, "the opposite of there" is still "the opposite of there." It's only when we start looking through the person in the mirror's pov that we will notice things like: hey, the wristwatch on my left hand is now on my right hand.

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Re: 1145: "Sky Color"

Postby marcgaston » Tue Dec 11, 2012 8:55 pm UTC

Although I have a PhD in theoretical physics, I have never felt to really understand the blue sky. Also the question with the mirror has intrigued me for years and all the answers people gave me never satisfied me completely. I felt quite stupid that after all my schooling I failed to understand these two effects.
When I read this discussion going on here I come to the conclusion that these questions are not so easy after all. I have read many good explanations but none of them convinced me 100% !
Many thanks guys!

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Re: 1145: "Sky Color"

Postby bmonk » Tue Dec 11, 2012 9:40 pm UTC

sameaspassword wrote:Actually, it is quite sad to see that so far apparantly hardly anyone has even understood the mirror reversal problem. It's quite obvious that a mirror placed at [imath]y=0[/imath] performs the operation [imath]y ↦ -y[/imath]. This is equivalent to a 180 degree rotation about the [imath]z[/imath] axis (up and down), except for the fact that such a rotation also changes [imath]x[/imath] to [imath]-x[/imath] (left and right). So, by mirroring the [imath]x[/imath] axis, we obtain the correct results and therefore all text in the mirror appears switched left and right. Problem solved?

Not quite. The mirror only breaks the symmetry along the [imath]y[/imath] axis, so left/right and up/down should still be symmetrical. And indeed, we can perform the mirroring also by rotating around the [imath]x[/imath] axis, which will result in top and bottom being switched. Therefore, the mirror actually flips top/down the same way it flips left/right because the results are completely identical!

Now, what's the resolution? The best explanation that I've come across is that we sort of expect things to be more or less to be symmetric w.r.t. to left and right, and asymmetric w.r.t. to top and bottom (gravity is a natural explanation for the expected symmetry breaking along the [imath]z[/imath] axis). Therefore, it is mentally easier for us to think in terms of the first transformation with the [imath]x[/imath] axis being reversed, as it involves only familiar looking objects. But I'm open to better explanations.


You are right--the mirror reflects the axis perpendicular to the mirror, here the y axis.

Then we try to map the real object onto the reflection. In our experience, rotation around a vertical axis is easy--we turn in a circle. But a horizontal axis is harder, unless we are doing somersaults. So we rotate the paper by a half-turn, switching both the y and x axes. Now the Y axis is switched twice. leaving it effectively unchanged, but the x axis is reversed. So we say mirrors flip left-right.

This is why a mirror on the ceiling makes things look so strange. We can no longer rotate objects on a vertical axis to have them "look right".
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Re: 1145: "Sky Color"

Postby garo » Tue Dec 11, 2012 10:26 pm UTC

I am surprised nobody mentioned a lake as a mirror. There's an up/down reversal. Or place a mirror on the floor and put a book next to it. And if you are feeling really evil try a sufficiently concave mirror.

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Re: 1145: "Sky Color"

Postby Moonfish » Tue Dec 11, 2012 11:30 pm UTC

Hi there Jorpho, Pfhorrest, domenox, addams and orthogon,

The sub-topic about the joy of teaching children things is rather important to me.
I was wondering if it would be acceptable for me to reintroduce it to the thread after the light and mirror sub-topics have played out?

Moonfish wrote:I’m going to tell him I really want to be his dad and I’m going to ask him if he would like that too.


Alternately, it could possibly be folded into the romantic poem sub-topic?

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Re: 1145: "Sky Color"

Postby severach » Wed Dec 12, 2012 2:45 am UTC

Many here are so close to solving the mirror problem and one pretty much has it with an incomplete explanation.

The mirror doesn't reverse anything. YOU DO!

Start with the object facing you. In desperation to get the side facing you to reflect in the mirror you flip it around, probably preferring horizontally because your eyes are horizontal. Like a typical human you forget what you did a few seconds ago and blame the mirror for something it didn't do.

Do it slowly and you can watch it happen right before your eye.

For all you nitwits trying to invoke some feel good Z axis reversal explanation (including someone in a video who's name won't be mentioned to protect the clever), pay attention while rotating. Notice that for you chumps stuck in 3D space two axes are being rotated while one is fixed. How did the Z axis get reversed? Oh look! YOU DID THAT TOO. So why don't you see it?

It's OK. You can blame the mirror for Z if you want to. The mirror does reverse the Z axis but it's not why the X axis is reversed. The mirror reversing the Z axis undoes what you did to the Z axis which makes you think that, like the Y axis, nothing happened to the Z axis when in fact a lot happened.

Y axis: didn't get rotated because humans are dumb like that

Z axis: got rotated and rotated back by the mirror

X axis: got rotated leaving the child wondering why when 2 axes are reversed that only one appears to change

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Re: 1145: "Sky Color"

Postby phlip » Wed Dec 12, 2012 3:04 am UTC

Well, I guess, congratulations for saying something that's been said many times in the thread already, while being condescending to everyone else for not saying it first?

Plus coming up with some nonsense about the reason you rotate things around the vertical axis being that "your eyes are horizontal"... Gold star.

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Re: 1145: "Sky Color"

Postby ike » Wed Dec 12, 2012 5:04 am UTC

honnza wrote:
marsgreekgod wrote:So my question is, if we got a very strong purple light in space, could we turn the sky purple?


Sure. This would also cause global warming, so beware.


No. Purple can only be produced by mixing blue and red (or other pairings of high and low frequency lights), a bright violet light could turn the sky violet, but the high and low frequency components of a purple light would scatter very differently so only one would predominate (most likely, the blue).

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Re: 1145: "Sky Color"

Postby Jorpho » Wed Dec 12, 2012 5:43 am UTC

ike wrote:
honnza wrote:
marsgreekgod wrote:So my question is, if we got a very strong purple light in space, could we turn the sky purple?
Sure. This would also cause global warming, so beware.
No. Purple can only be produced by mixing blue and red (or other pairings of high and low frequency lights), a bright violet light could turn the sky violet, but the high and low frequency components of a purple light would scatter very differently so only one would predominate (most likely, the blue).
What makes you say that? There's plainly a purple component of the electromagnetic spectrum that exists independently of the blue components and red components.

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Re: 1145: "Sky Color"

Postby ealloc » Wed Dec 12, 2012 5:46 am UTC

Here's my shot at the blue sky, and why it blue and not green or violet:

First, note that the sun is a very hot black body and appears white to us. Black bodies start off red, then warm up to orange, then yellow, then white. In each step we are adding more color components and get closer and closer to white light. Note that black bodies are never green (or blue), even though there are blue and green color components in the black body light.

Now, when the sunlight is scattered by the sky, the different color components get scattered as lambda^4. The original black body radiation from the sunlight is roughly a gaussian centered at 550nm, and it turns out that if you weight the gaussian by lambda^4, you get a new roughly gaussian shape shifted up to blue a little bit. Type this into wolfram alpha to see: plot e^(-((x-550)^2)/(2*100^2)), e^(-((x-550)^2)/(2*100^2))*(x/600)^4

You can see this precisely for the sun spectrum by looking up the image SunvsSky.jpg in google images (sorry, I'm not allowed to have links here yet).
So effectively the red part of the spectrum gets lost, but the blue and green (and violet!) parts are still there. You get white light, minus the red. Now go into any painting program, and draw the color that is 'white minus the red'. ie, take 0xffffff and turn it into 0x00ffff. (or maybe just remove some of the red, eg 0xaaffff) Tada, it's light blue.

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Re: 1145: "Sky Color"

Postby phlip » Wed Dec 12, 2012 5:56 am UTC

Jorpho wrote:
ike wrote:No. Purple can only be produced by mixing blue and red (or other pairings of high and low frequency lights), a bright violet light could turn the sky violet, but the high and low frequency components of a purple light would scatter very differently so only one would predominate (most likely, the blue).
What makes you say that? There's plainly a purple component of the electromagnetic spectrum that exists independently of the blue components and red components.

Violet is a colour that exists on the spectrum, but purple is not. Wikipedia has details.

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Re: 1145: "Sky Color"

Postby Jorpho » Wed Dec 12, 2012 6:10 am UTC

Okay, there's a violet component of the spectrum that exists independently of the blue and red components.

But I guess that was not technically the question.

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Re: 1145: "Sky Color"

Postby Harry Voyager » Wed Dec 12, 2012 7:02 am UTC

What I would like to know is why we perceive violet and purple as mostly the same, even when they are, in fact, significantly different colours.

See, one of my other hobbies is fountain pens, and I've run into this glorious violet ink, which, in person, is an extreme blue, and, on camera is poipul

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Re: 1145: "Sky Color"

Postby honnza » Wed Dec 12, 2012 7:07 am UTC

ike wrote:
honnza wrote:
marsgreekgod wrote:So my question is, if we got a very strong purple light in space, could we turn the sky purple?


Sure. This would also cause global warming, so beware.


No. Purple can only be produced by mixing blue and red (or other pairings of high and low frequency lights), a bright violet light could turn the sky violet, but the high and low frequency components of a purple light would scatter very differently so only one would predominate (most likely, the blue).


You can anticipate the scattering, and shine a mostly-red light. If the sky scatters 50% of red light but no blue, you can shine a [imath]2R+B[/imath] light onto the sky to get a [imath]R+B[/imath] (magenta or purple) sky.

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Re: 1145: "Sky Color"

Postby Pfhorrest » Wed Dec 12, 2012 7:35 am UTC

Moonfish wrote:Hi there Jorpho, Pfhorrest, domenox, addams and orthogon,

The sub-topic about the joy of teaching children things is rather important to me.
I was wondering if it would be acceptable for me to reintroduce it to the thread after the light and mirror sub-topics have played out?

Moonfish wrote:I’m going to tell him I really want to be his dad and I’m going to ask him if he would like that too.


Alternately, it could possibly be folded into the romantic poem sub-topic?

I'm not sure why you're asking us, since none of us are admins, but as far as I can tell the teaching-children topic is on topic for the comic, and it's not like other threads haven't completely derailed into things that had nothing to do with the topic before, so even if it wasn't on-topic it would be fine to tangent over to it if that's the direction the conversation goes. I saw your earlier post on the subject and just didn't have anything worthwhile to say in response; if anyone else does, that might get a subthread rolling, but if not, well... maybe everyone just agrees with you and doesn't have anything else to say?

Good luck with your lady-friend and her son, too!
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"I am Sam. Sam I am. I do not like trolls, flames, or spam."
The Codex Quaerendae (my philosophy) - The Chronicles of Quelouva (my fiction)

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Re: 1145: "Sky Color"

Postby orthogon » Wed Dec 12, 2012 9:28 am UTC

Harry Voyager wrote:What I would like to know is why we perceive violet and purple as mostly the same, even when they are, in fact, significantly different colours.

See, one of my other hobbies is fountain pens, and I've run into this glorious violet ink, which, in person, is an extreme blue, and, on camera is poipul


I believe that this is because the response of the red cone has a recovery at short wavelengths, around the violet. This figure shows a hint of the effect, although the red curve stops around the crucial wavelengths:

Image

Hence the response to magenta (red+blue) is similar to the response to violet, even though they are quite different colours.
xtifr wrote:... and orthogon merely sounds undecided.

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Re: 1145: "Sky Color"

Postby stianhat » Wed Dec 12, 2012 12:56 pm UTC

stianhat wrote:The sky is blue because of the colors it *lacks* not the colors it has - we percieve chlorophyll filled leaves as green because they lack the red light and correspondingly, as someone was close to earlier, we see the sky as blue because it lacks red light, but also has violet light ( violet = not yellow ) (not red = green). the resultant mix of (not yellow) + green = blue. The blue light that is already there just adds to this.
Chlorophyll absorbs red and violet light such that the only light it reflects is green, which we perceive. But the sky is blue because the blue end of the spectrum is more widely scattered than the other wavelengths. The color wheel doesn't really get into it, I think.
[/quote]

No, its not about the color wheel, thanks for that straw man, it is about the brain interpreting the signal from your optical nerves. The brain will interpret the lack of a color as the reciprocal of that color. Dont believe me? Stare at something intensely colored without moving your focal point or shifting your view. 15-20 seconds should do it, now look at a white paper.

If you are a regular non-super-human you should see an outline of the object you were looking at - just in the reciprocal color. Same thing happens when you look at a bright light at an otherwise dark night and then close your eyes. you'll see artifacts (I call em "stars") when your eyes are closed. What color are these stars? ... Exactly. The reason for this is that your photoreceptor cells are overloaded and don't "report in" as often if you have supersaturated them, the lack of signal is interpreted as a color. it isnt a physical phenomenon (well, in the end, everything is, but you know what I mean) it is a neurological / signal interpretation issue.

This means that the sky will *look blue*, even when its spectrum says it should be more violet because your brain adds the "lack of yellow" to the "blue".

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Re: 1145: "Sky Color"

Postby Jorpho » Wed Dec 12, 2012 2:25 pm UTC

stianhat wrote:If you are a regular non-super-human you should see an outline of the object you were looking at - just in the reciprocal color. Same thing happens when you look at a bright light at an otherwise dark night and then close your eyes. you'll see artifacts (I call em "stars") when your eyes are closed. What color are these stars? ... Exactly. The reason for this is that your photoreceptor cells are overloaded and don't "report in" as often if you have supersaturated them, the lack of signal is interpreted as a color. it isnt a physical phenomenon (well, in the end, everything is, but you know what I mean) it is a neurological / signal interpretation issue.
You propose that because overstimulated photoreceptor cells interpret a lack of signal as a color, that under normal conditions the interpretation of a color inherently requires the lack of a signal?

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Re: 1145: "Sky Color"

Postby addams » Wed Dec 12, 2012 4:24 pm UTC

Jorpho wrote:Why stop there when you can simply say "Because God made it that way" ? :P

Yes. God made the sky to vibrate Red.
It is the primate beast that sees it Blue.
The Red apple is vibrating Blue.
eeww.

That part seems easy.
The mirror shit makes me uncomfortable.
Waa. I don't understand!

I blame Alice in Wonderland.
Would you step through the mirror?

Poor Alice. What a night mare.
The way I heard the story, she woke up safe and sound.

Is Alice the same girl that went down a Rabbit Hole?
Pfft. I've been avoiding mirrors, like a vampire.
Excetric? Oh, I don't know how mirrors work.
Life is, just, an exchange of electrons; It is up to us to give it meaning.

We are all in The Gutter.
Some of us see The Gutter.
Some of us see The Stars.
by mr. Oscar Wilde.

Those that want to Know; Know.
Those that do not Know; Don't tell them.
They do terrible things to people that Tell Them.

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Re: 1145: "Sky Color"

Postby orthogon » Wed Dec 12, 2012 4:50 pm UTC

addams wrote:I blame Alice in Wonderland.
Would you step through the mirror?


Probably not. If I remember my A-level chemistry correctly, there would be serious risks: lots of food would be indigestible because it contained the wrong handedness of optical isomer, and I fear (though less sure about this one) that I would have no immunity to the mirror-image viruses and bacteria for similar reasons.

I never read Alice through the Looking Glass, and people don't talk about it as much as Alice in Wonderland. Presumably it was the usual substandard-sequel story?
xtifr wrote:... and orthogon merely sounds undecided.

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Re: 1145: "Sky Color"

Postby Aelfyre » Wed Dec 12, 2012 5:13 pm UTC

orthogon wrote:
addams wrote:I blame Alice in Wonderland.
I never read Alice through the Looking Glass, and people don't talk about it as much as Alice in Wonderland. Presumably it was the usual substandard-sequel story?


I've read it, but it has been decades..
Spoiler:
I'm not sure that it is a chronological prequel/sequel of any mind. From what I recall its a short story where she fall asleep and has a dream about going thru the looking glass and discovering (as she suspected) that not everything on the other side is the same.. just the stuff she could see. (so if you go thru a mirror and then walk thru a door to a room you couldn't see from the original side of the mirror, things would be different.. again it was a dream scape) In this book the Red and White queens were representations of her cats.
Xanthir wrote:To be fair, even perfectly friendly antimatter wildebeests are pretty deadly.

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Re: 1145: "Sky Color"

Postby domenox » Wed Dec 12, 2012 5:34 pm UTC

Through the Looking Glass is actually about as good and just as long as Alice in Wonderland. Personally, I think it's a better story.

Presumably it's not as popular as Alice in Wonderland because in animated/live adaptations, elements from it are usually mixed in with the Wonderland elements and the whole thing is simply referred to as Alice in Wonderland. For example, the Red Queen, the Jabberwocky, Twiddledee and Twiddledum all come from Through the Looking Glass.


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